fix: serializes an empty object to be compliant with the specification

Signed-off-by: Vincent Biret <[email protected]>

Author: baywet

src/Microsoft.OpenApi/Models/OpenApiHeader.cs

@@ -85,15 +85,33 @@ internal OpenApiHeader(IOpenApiHeader header)
         /// </summary>
         public void SerializeAsV31(IOpenApiWriter writer)
         {
-            SerializeInternal(writer, OpenApiSpecVersion.OpenApi3_0, (writer, element) => element.SerializeAsV31(writer), (writer, element) => element?.SerializeAsV31(writer));
+            SerializeInternal(writer, OpenApiSpecVersion.OpenApi3_0, (writer, element) => element.SerializeAsV31(writer), (writer, element) => 
+            {
+              if(element is null)
+              {
+                writer.WriteStartObject();
+                writer.WriteEndObject();
+              }
+              else
+                element.SerializeAsV31(writer);
+            });
         }
 
         /// <summary>
         /// Serialize <see cref="OpenApiHeader"/> to Open Api v3.0
         /// </summary>
         public void SerializeAsV3(IOpenApiWriter writer)
         {
-            SerializeInternal(writer, OpenApiSpecVersion.OpenApi3_0, (writer, element) => element.SerializeAsV3(writer), (writer, element) => element?.SerializeAsV3(writer));
+            SerializeInternal(writer, OpenApiSpecVersion.OpenApi3_0, (writer, element) => element.SerializeAsV3(writer), (writer, element) => 
+            {
+              if(element is null)
+              {
+                writer.WriteStartObject();
+                writer.WriteEndObject();
+              }
+              else
+                element.SerializeAsV3(writer);
+            });
         }
 
         internal void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version, 

src/Microsoft.OpenApi/Models/OpenApiParameter.cs

@@ -102,13 +102,31 @@ internal OpenApiParameter(IOpenApiParameter parameter)
         /// <inheritdoc/>
         public void SerializeAsV31(IOpenApiWriter writer)
         {
-            SerializeInternal(writer, OpenApiSpecVersion.OpenApi3_1, (writer, element) => element.SerializeAsV31(writer), (writer, element) => element?.SerializeAsV31(writer));
+            SerializeInternal(writer, OpenApiSpecVersion.OpenApi3_1, (writer, element) => element.SerializeAsV31(writer), (writer, element) => 
+            {
+              if(element is null)
+              {
+                writer.WriteStartObject();
+                writer.WriteEndObject();
+              }
+              else
+                element.SerializeAsV31(writer);
+            });
         }
 
         /// <inheritdoc/>
         public void SerializeAsV3(IOpenApiWriter writer)
         {
-            SerializeInternal(writer, OpenApiSpecVersion.OpenApi3_0, (writer, element) => element.SerializeAsV3(writer), (writer, element) => element?.SerializeAsV3(writer));
+            SerializeInternal(writer, OpenApiSpecVersion.OpenApi3_0, (writer, element) => element.SerializeAsV3(writer), (writer, element) => 
+            {
+              if(element is null)
+              {
+                writer.WriteStartObject();
+                writer.WriteEndObject();
+              }
+              else
+                element.SerializeAsV3(writer);
+            });
         }
 
         internal void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version, 

src/Microsoft.OpenApi/Models/OpenApiRequestBody.cs

@@ -52,15 +52,33 @@ internal OpenApiRequestBody(IOpenApiRequestBody requestBody)
         /// </summary>
         public void SerializeAsV31(IOpenApiWriter writer)
         {
-            SerializeInternal(writer, OpenApiSpecVersion.OpenApi3_1, (writer, element) => element?.SerializeAsV31(writer));
+            SerializeInternal(writer, OpenApiSpecVersion.OpenApi3_1, (writer, element) => 
+            {
+              if(element is null)
+              {
+                writer.WriteStartObject();
+                writer.WriteEndObject();
+              }
+              else
+                element.SerializeAsV31(writer);
+            });
         }
 
         /// <summary>
         /// Serialize <see cref="OpenApiRequestBody"/> to Open Api v3.0
         /// </summary>
         public void SerializeAsV3(IOpenApiWriter writer)
         {
-            SerializeInternal(writer, OpenApiSpecVersion.OpenApi3_0, (writer, element) => element?.SerializeAsV3(writer));
+            SerializeInternal(writer, OpenApiSpecVersion.OpenApi3_0, (writer, element) => 
+            {
+              if(element is null)
+              {
+                writer.WriteStartObject();
+                writer.WriteEndObject();
+              }
+              else
+                element.SerializeAsV3(writer);
+            });
         }
 
         internal void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version,

src/Microsoft.OpenApi/Models/OpenApiResponse.cs

@@ -53,15 +53,33 @@ internal OpenApiResponse(IOpenApiResponse response)
         /// </summary>
         public void SerializeAsV31(IOpenApiWriter writer)
         {
-            SerializeInternal(writer, OpenApiSpecVersion.OpenApi3_1, (writer, element) => element.SerializeAsV31(writer), (writer, element) => element?.SerializeAsV31(writer));
+            SerializeInternal(writer, OpenApiSpecVersion.OpenApi3_1, (writer, element) => element.SerializeAsV31(writer), (writer, element) =>
+            {
+              if(element is null)
+              {
+                writer.WriteStartObject();
+                writer.WriteEndObject();
+              }
+              else
+                element.SerializeAsV31(writer);
+            });
         }
 
         /// <summary>
         /// Serialize <see cref="OpenApiResponse"/> to Open Api v3.0.
         /// </summary>
         public void SerializeAsV3(IOpenApiWriter writer)
         {
-            SerializeInternal(writer, OpenApiSpecVersion.OpenApi3_0, (writer, element) => element.SerializeAsV3(writer), (writer, element) => element?.SerializeAsV3(writer));
+            SerializeInternal(writer, OpenApiSpecVersion.OpenApi3_0, (writer, element) => element.SerializeAsV3(writer), (writer, element) =>
+            {
+              if(element is null)
+              {
+                writer.WriteStartObject();
+                writer.WriteEndObject();
+              }
+              else
+                element.SerializeAsV3(writer);
+            });
         }
 
         private void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version,