Ensure the default key is set before we try accessing it

[dkotter]

Description of the Change

When we get our prompts, we check which one is marked as the default. If we try and do this before prompts have been saved though, the default key isn't set and will result in an undefined index warning. This PR addresses that by checking to ensure that key exists before we access it.

Closes #648

How to test the Change

1. On a new install, install ClassifAI from the develop branch
2. Go to the Site Health screen (/wp-admin/site-health.php?tab=debug)
3. You should see a PHP notice
4. Checkout this PR and run the same steps
5. The notice should no longer show

Changelog Entry

Fixed - Check for the default array key before we access it

Credits

Props @dkotter, @Sidsector9

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.