assistant: render api key editor if no credentials are set (#7197)

This hopefully reduces confusion for new users. I updated the docs just
this morning, but I figured it's probably better to fix the issue
itself.

So what this does is to render the API key editor whenever the assistant
panel is opened/focused and no credentials can be found.

See: https://github.com/zed-industries/zed/discussions/6943

Release Notes:

- Fixed assistant panel not showing dialog to enter API key when opened
without saved credentials.

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Co-authored-by: Piotr <piotr@zed.dev>