Fix tests

Author: Kludex

pydantic_ai_slim/pydantic_ai/_agent_graph.py

@@ -175,7 +175,7 @@ async def _prepare_messages(
         else:
             parts.extend(await self._sys_parts(run_context))
 
-        if user_prompt:
+        if user_prompt is not None:
             parts.append(_messages.UserPromptPart(user_prompt))
         return messages, _messages.ModelRequest(parts)