fix: preserve empty-string text parts in A2A converter

Part(text='') is valid — it is produced by code_execution_utils.py when
code execution returns None, and by interactions_utils.py when the
Interactions API returns None text. Gemini 2.5 Flash thinking mode also
emits empty text parts.

The truthiness check `if part.text:` treats '' as falsy, causing the
converter to skip the part entirely. When all parts in a response are
empty the A2A message ends up with zero parts and the client sees
"broken thinking" with no content.

Change the check to `if part.text is not None:` so that empty strings
are correctly wrapped as TextPart while None is still skipped.

Add a regression test for the empty-string case.

Fixes #5341

Author: voidborne-d

src/google/adk/a2a/converters/part_converter.py

@@ -179,7 +179,7 @@ def convert_genai_part_to_a2a_part(
 ) -> Optional[a2a_types.Part]:
   """Convert a Google GenAI Part to an A2A Part."""
 
-  if part.text:
+  if part.text is not None:
     a2a_part = a2a_types.TextPart(text=part.text)
     if part.thought is not None:
       a2a_part.metadata = {_get_adk_metadata_key('thought'): part.thought}

tests/unittests/a2a/converters/test_part_converter.py

@@ -297,6 +297,23 @@ def test_convert_text_part_with_thought(self):
     assert result.root.metadata is not None
     assert result.root.metadata[_get_adk_metadata_key("thought")]
 
+  def test_convert_empty_text_part(self):
+    """Test that Part(text='') is preserved, not dropped.
+
+    Regression test for #5341: empty-string text parts are valid and
+    must not fall through to the unsupported-part warning.
+    """
+    # Arrange
+    genai_part = genai_types.Part(text="")
+
+    # Act
+    result = convert_genai_part_to_a2a_part(genai_part)
+
+    # Assert — should produce a valid TextPart, not None
+    assert result is not None
+    assert isinstance(result.root, a2a_types.TextPart)
+    assert result.root.text == ""
+
   def test_convert_file_data_part(self):
     """Test conversion of GenAI file_data Part to A2A Part."""
     # Arrange