Do not allow blank empty content to be sent to Anthropic Claude LLM

[mike-r-mclaughlin]

Description

In some cases, an empty message is sent to the LLM but Anthropic's Claude does not allow that. It returns messages.12: all messages must have non-empty content except for the optional final assistant message error.

2025-01-21 15:40:05,457 - ERROR livekit.agents.pipeline - Error in _stream_synthesis_task
Traceback (most recent call last):
  File "/Users/noah/livekit-cartesia-claude-deepgram/venv/lib/python3.13/site-packages/livekit/agents/utils/log.py", line 16, in async_fn_logs
    return await fn(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/noah/livekit-cartesia-claude-deepgram/venv/lib/python3.13/site-packages/livekit/agents/pipeline/agent_output.py", line 273, in _stream_synthesis_task
    async for seg in tts_source:
    ...<9 lines>...
        tts_stream.push_text(seg)
  File "/Users/noah/livekit-cartesia-claude-deepgram/venv/lib/python3.13/site-packages/livekit/agents/utils/aio/itertools.py", line 47, in tee_peer
    item = await iterator.__anext__()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/noah/livekit-cartesia-claude-deepgram/venv/lib/python3.13/site-packages/livekit/agents/pipeline/pipeline_agent.py", line 1055, in _llm_stream_to_str_generator
    async for chunk in stream:
    ...<7 lines>...
        yield content
  File "/Users/noah/livekit-cartesia-claude-deepgram/venv/lib/python3.13/site-packages/livekit/agents/llm/llm.py", line 239, in __anext__
    raise exc from None
  File "/Users/noah/livekit-cartesia-claude-deepgram/venv/lib/python3.13/site-packages/livekit/agents/llm/llm.py", line 149, in _main_task
    return await self._run()
           ^^^^^^^^^^^^^^^^^
  File "/Users/noah/livekit-cartesia-claude-deepgram/venv/lib/python3.13/site-packages/livekit/plugins/anthropic/llm.py", line 234, in _run
    raise APIStatusError(
    ...<4 lines>...
    )
livekit.agents._exceptions.APIStatusError: Error code: 400 - {'type': 'error', 'error': {'type': 'invalid_request_error', 'message': 'messages.12: all messages must have non-empty content except for the optional final assistant message'}} {"pid": 9123, "job_id": "AJ_zFDs8T7P9A59"}

How to reproduce

It seems to be more easily reproducible when there is a lot of background noise. See details in this Slack thread.

[davidzhao]

fixed by #1410