fix: Raise exception when tool call result is None (#1253)

# What does this PR do?

When there are issues with the tool call function, an exception is
raised but the error message is not informative. This adds a clearer
message to tell users to check their functions.
```
Traceback (most recent call last):
  File "/Users/phayes/projects/llama-stack/llama-stack/llama_stack/distribution/server/server.py", line 208, in sse_generator
    async for item in event_gen:
  File "/Users/phayes/projects/llama-stack/llama-stack/llama_stack/providers/inline/agents/meta_reference/agents.py", line 165, in _create_agent_turn_streaming
    async for event in agent.create_and_execute_turn(request):
  File "/Users/phayes/projects/llama-stack/llama-stack/llama_stack/providers/inline/agents/meta_reference/agent_instance.py", line 197, in create_and_execute_turn
    async for chunk in self.run(
  File "/Users/phayes/projects/llama-stack/llama-stack/llama_stack/providers/inline/agents/meta_reference/agent_instance.py", line 389, in run
    async for res in self._run(
  File "/Users/phayes/projects/llama-stack/llama-stack/llama_stack/providers/inline/agents/meta_reference/agent_instance.py", line 811, in _run
    content=tool_result.content,
AttributeError: 'NoneType' object has no attribute 'content'
```

## Test Plan

Ran the same script and exception is raised with clearer error message.

Signed-off-by: Yuan Tang <[email protected]>

llama_stack/providers/inline/agents/meta_reference/agent_instance.py

@@ -804,6 +804,10 @@ async def _run(
                         toolgroup_args,
                         tool_to_group,
                     )
+                    if tool_result.content is None:
+                        raise ValueError(
+                            f"Tool call result (id: {tool_call.call_id}, name: {tool_call.tool_name}) does not have any content"
+                        )
                     result_messages = [
                         ToolResponseMessage(
                             call_id=tool_call.call_id,