[BUG] Don't force compulsory API Key in OpenAI-compatible provider

[telefrancisco]

Describe the bug
Some OpenAI-compatible providers do their authentication via headers instead of an API Key, and some others do not require any authentication at all.
Due to compulsory API Key, it's impossible to use these providers.

To Reproduce
Steps to reproduce the behavior:

1. Go to "Configuration/Providers"
2. Select "OpenAI Compatible"
3. Leave empty "API Key"
4. It's impossible to save because it requires a non-empty API Key

Expected behavior
Empty "API Key" should allow for saving the provider.

What version of zoo are you running
Version: 3.79.100397 (1ad8f52)