fix: guard against Content with no parts in _content_to_message_param

Merge https://github.com/google/adk-python/pull/6312

Avoid raising TypeError when types.Content has parts=None or parts=[] in LiteLLM adapter.

PiperOrigin-RevId: 967357294

src/google/adk/models/lite_llm.py

@@ -1233,7 +1233,7 @@ async def _content_to_message_param(
     *,
     provider: str = "",
     model: str = "",
-) -> Union[Message, list[Message]]:
+) -> Union[Message, list[Message]] | None:
   """Converts a types.Content to a litellm Message or list of Messages.
 
   Handles multipart function responses by returning a list of
@@ -1245,14 +1245,18 @@ async def _content_to_message_param(
     model: The LiteLLM model string, used for provider-specific behavior.
 
   Returns:
-    A litellm Message, a list of litellm Messages.
+    A litellm Message, a list of litellm Messages, or None if skipped.
   """
   _ensure_litellm_imported()
 
+  # Skip content if there are no parts to avoid LiteLLM adapter errors.
+  parts = content.parts or []
+  if not parts:
+    return None
+
   tool_messages: list[Message] = []
   non_tool_parts: list[types.Part] = []
-  content_parts_or_empty = content.parts or []
-  for part in content_parts_or_empty:
+  for part in parts:
     if part.function_response:
       function_response = part.function_response
       response = function_response.response
@@ -1300,7 +1304,7 @@ async def _content_to_message_param(
   role = _to_litellm_role(content.role)
 
   if role == "user":
-    user_parts = [part for part in content_parts_or_empty if not part.thought]
+    user_parts = [part for part in parts if not part.thought]
     message_content = (
         await _get_content(user_parts, provider=provider, model=model) or None
     )
@@ -1312,7 +1316,7 @@ async def _content_to_message_param(
     tool_calls: list[_OutboundToolCall] = []
     content_parts: list[types.Part] = []
     reasoning_parts: list[types.Part] = []
-    for part in content_parts_or_empty:
+    for part in parts:
       if part.function_call:
         function_call = part.function_call
         if not function_call.name:

tests/unittests/models/test_litellm.py

@@ -2085,6 +2085,23 @@ async def test_content_to_message_param_user_message():
   assert message["content"] == "Test prompt"
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize("parts", [None, []])
+async def test_content_to_message_param_user_message_without_parts(parts):
+  # parts must not raise.
+  content = types.Content(role="user", parts=parts)
+  message = await _content_to_message_param(content)
+  assert message is None
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("parts", [None, []])
+async def test_content_to_message_param_assistant_message_without_parts(parts):
+  content = types.Content(role="assistant", parts=parts)
+  message = await _content_to_message_param(content)
+  assert message is None
+
+
 @pytest.mark.asyncio
 @pytest.mark.parametrize("file_uri,mime_type", FILE_URI_TEST_CASES)
 async def test_content_to_message_param_user_message_with_file_uri(