Fix edge case with leading line ending in 'split_lines'.

src/prompt_toolkit/completion/nested.py

@@ -69,7 +69,7 @@ def from_nested_dict(cls, data: NestedDict) -> NestedCompleter:
             elif isinstance(value, dict):
                 options[key] = cls.from_nested_dict(value)
             elif isinstance(value, set):
-                options[key] = cls.from_nested_dict({item: None for item in value})
+                options[key] = cls.from_nested_dict(dict.fromkeys(value))
             else:
                 assert value is None
                 options[key] = None

src/prompt_toolkit/formatted_text/utils.py

@@ -89,8 +89,7 @@ def split_lines(
         parts = string.split("\n")
 
         for part in parts[:-1]:
-            if part:
-                line.append(cast(OneStyleAndTextTuple, (style, part, *mouse_handler)))
+            line.append(cast(OneStyleAndTextTuple, (style, part, *mouse_handler)))
             yield line
             line = []
 

tests/test_formatted_text.py

@@ -274,7 +274,7 @@ def test_split_lines_3():
     lines = list(split_lines([("class:a", "\n")]))
 
     assert lines == [
-        [],
+        [("class:a", "")],
         [("class:a", "")],
     ]
 
@@ -284,3 +284,15 @@ def test_split_lines_3():
     assert lines == [
         [("class:a", "")],
     ]
+
+
+def test_split_lines_4():
+    "Edge cases: inputs starting and ending with newlines."
+    # -1-
+    lines = list(split_lines([("class:a", "\nline1\n")]))
+
+    assert lines == [
+        [("class:a", "")],
+        [("class:a", "line1")],
+        [("class:a", "")],
+    ]