[FIX] [litellm_wrapper] omit empty text block for image-only vision calls

[ebarkhordar]

What

LiteLLM's vision message builders always prepended a text content block, even
when the task was empty or None. An image-only call such as llm.run("", img=...)
or run(None, img=...) therefore built a user message containing
{"type": "text", "text": ""} next to the image block.

Root cause

anthropic_vision_processing (both the direct-URL and base64 branches) and
openai_vision_processing unconditionally start the content list with
{"type": "text", "text": task}. When task is empty or None, that block is
empty.

Invariant

No content block in a vision message should carry empty text. The wrapper already
enforces this for system blocks in _prepare_messages, which drops empty system
blocks with the note that Anthropic rejects them ("system: text content blocks
must be non-empty"). The vision user-text block was the one spot in the same
family that was not guarded.

Fix

Add a small _vision_content(task, image_block) helper that includes the text
block only when task is a non-empty (non-whitespace) string, then route all
three construction sites through it. With no task the message carries just the
image block; with a task, behavior is unchanged. This mirrors the existing
empty-system-block normalization.

Verification

Ran in a clean Docker container (python:3.12-slim, litellm==1.76.1), offline
(--network none), against this branch and against master:

• New test tests/utils/test_litellm_vision_empty_text.py: the 7 image-only
  cases (empty / whitespace / None task, Anthropic and OpenAI paths, plus a
  forced direct-URL case) FAIL on master (the empty {"type": "text", "text": ""}
  block is present) and PASS on this branch.
• 2 over-deletion guards (a real task keeps its text block) pass on both sides,
  so the fix does not drop legitimate text.

docker run --rm --network none -e PYTHONPATH=/src -e WORKSPACE_DIR=/tmp \
  -v <clone>:/src -w /src <image> \
  python -m pytest tests/utils/test_litellm_vision_empty_text.py -v

The tests assert the message structure the wrapper builds. I did not make a live
Anthropic API call, so I have not personally observed the rejection; the premise
rests on Anthropic's documented "text content blocks must be non-empty"
constraint and on this repo's own existing system-block guard for it.

Black (24.2.0) and Ruff (0.2.1) are clean on both changed files.

Prepared with AI assistance; reproduced and verified as described above.

[github-actions]

Hello there, thank you for opening an PR ! 🙏🏻 The team was notified and they will get back to you asap.

[ebarkhordar]

No rush on this. It is a small litellm_wrapper fix (drop the empty text block for image-only vision calls, since the provider rejects a bare empty content block), and I am happy to rebase or add a test if that would help you review.

[ebarkhordar]

Closing this to keep my open PR list at a size I can actually shepherd. Nothing here is
blocked on anyone and this is not a nudge.

The change still applies if it is useful later, and I am happy to reopen and rebase it
at any point. The branch stays up so reopening is a click.