Fix empty list in content on tool calls causing ollama parse error (#102)

This is related to google/adk-python#49

src/google/adk/models/lite_llm.py

@@ -178,7 +178,7 @@ def _content_to_message_param(
 
     return ChatCompletionAssistantMessage(
         role=role,
-        content=_get_content(content.parts),
+        content=_get_content(content.parts) or None,
         tool_calls=tool_calls or None,
     )