check thinking is non-null in anthropic messages

[gswangg]

Description

extended thinking was mostly fixed by #18834, though claude 4 will also occasionally issue tool calls with no accompanying text, and null 'thinking' in additional_kwargs, as seen below:

DEBUG:    Empty response full structure: {
  "role": "assistant",
  "additional_kwargs": {
    "tool_calls": [
      {
        #[redacted]
      }
    ],
    "thinking": null
  },
  "blocks": [
    {
      "block_type": "text",
      "text": ""
    }
  ]
}

this causes a downstream exception:
anthropic.BadRequestError: Error code: 400 - {'type': 'error', 'error': {'type': 'invalid_request_error', 'message': 'messages: text content blocks must be non-empty'}}

Fixes # (issue)

Version Bump?

Did I bump the version in the pyproject.toml file of the package I am updating? (Except for the llama-index-core package)

• Yes
• No

Type of Change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Your pull-request will likely not be merged unless it is covered by some form of impactful unit testing.

• I tested locally and checked that this resolved the bug I was encountering.

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added Google Colab support for the newly added notebooks.
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I ran uv run make format; uv run make lint to appease the lint gods