assistant: Only push text content if not empty with image content (#16270)

If you submit an image with empty space above it and text below, it will
fail with this error:


![image](https://github.com/user-attachments/assets/a4a2265e-815f-48b5-b09e-e178fce82ef7)

Now instead it fails with an error about needing a message.

<img width="640" alt="image"
src="https://github.com/user-attachments/assets/72b267eb-b288-40a5-a829-750121ff16cc">

It will however work with text above and empty text below the image now.

Release Notes:

- Improved conformance with Anthropic Images in Chat Completions API