fix: preserve empty-string text parts in A2A converter

Merge #5343

## Summary

Fixes #5341 — `convert_genai_part_to_a2a_part()` drops `Part(text='')`  because the truthiness check `if part.text:` treats empty strings as falsy.

## Root Cause

`Part(text='')`  is valid and is produced in at least two places in the ADK:

1. **`code_executors/code_execution_utils.py`** — when code execution completes with `None` output
2. **`models/interactions_utils.py`** — when the Interactions API returns `None` text content

Gemini 2.5 Flash (thinking mode) also emits empty text parts.

When the converter drops all parts the A2A message ends up with zero parts and the client sees "broken thinking" with no content.

## Fix

Change line 182 from `if part.text:` to `if part.text is not None:` so that empty strings are correctly wrapped as `TextPart` while `None` is still skipped.

## Test

Added `test_convert_empty_text_part` — verifies that `Part(text='')`  produces a valid `TextPart(text='')`  instead of returning `None`.

COPYBARA_INTEGRATE_REVIEW=#5343 from voidborne-d:fix/part-converter-empty-text dbb2f20
PiperOrigin-RevId: 905370961

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