fix(sdk-python): guard assistant message with content check

Avoid emitting spurious empty assistant messages for tool-call-only
AIMessages (common with OpenAI models where content is empty string).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mxmzb

sdk-python/copilotkit/langgraph.py

@@ -163,11 +163,12 @@ def langchain_messages_to_copilotkit(
                 "id": message.id,
             })
         elif isinstance(message, AIMessage):
-            result.append({
-                "role": "assistant",
-                "content": content,
-                "id": message.id,
-            })
+            if content:
+                result.append({
+                    "role": "assistant",
+                    "content": content,
+                    "id": message.id,
+                })
             if message.tool_calls:
                 for tool_call in message.tool_calls:
                     result.append({