Cannot save via chat settings

[dlqqq]

Description

There is no way to save through the settings UI when any language/embedding model requires an API key. Bug was introduced by #918.

Screen.Recording.2024-08-05.at.4.40.58.PM.mov

Reproduce

1. Select any language model that requires an API key
2. Save settings
3. Try to save settings again

Expected behavior

Jupyter AI should work.

[dlqqq]

The issue is that the request payload sent to POST /ai/config always includes an empty API key value for API keys required by the current language/embedding model. An API key should be included in the request payload only when one is being created or updated.

I called this out in my review, but received pushback that I did not have time to verify, so I merged #918 anyways. Since this is my responsibility, I will open a PR for this.