Modify code generation to consider default: null as nullable

Author: yskn4492

src/openApi/v3/parser/getModel.ts

@@ -27,7 +27,7 @@ export const getModel = (
         deprecated: definition.deprecated === true,
         isDefinition,
         isReadOnly: definition.readOnly === true,
-        isNullable: definition.nullable === true || (!definition.nullable && definition.default === null),
+        isNullable: definition.nullable === true || (definition.nullable !== false && definition.default === null),
         isRequired: false,
         format: definition.format,
         maximum: definition.maximum,