fix(anthropic): handle empty AIMessage

[ccurme]

Claude can generate AIMessages with no content (including no tool calls), but these aren't valid inputs unless they end the sequence of messages (e.g., to pre-fill a response).

[codspeed-hq]

CodSpeed Performance Report

Merging #33390 will not alter performance

_{Comparing cc/anthropic_empty_message (efd7142) with master (d8a680e)}

Summary

✅ 3 untouched
⏩ 31 skipped¹

Footnotes

1. 31 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩