Multi-modal chat with `gemini-pro-vision`

[muddi900]

I am trying to build a multimodal chat component for my app. However, you can't have a text only input in 'gemini-pro-vision' ChatSession.

We should be able to have a chat that includes text-only input and multi-modal inputs.

[natebosch]

I believe this is a known issue, but I'm not aware of the specific plans for changes here. Questions about the backend and model behavior can be directed to https://discuss.ai.google.dev/

Closing this issue for now since this issue tracker relates to the Dart implementation.