AJAX: Fix issue if event categories are invalid

[r-a-y]

Hi Stephen,

This PR fixes an issue when fetching events via AJAX if the event categories are invalid.

It basically mirrors what you have already done with event tags. Probably just forgot to do the same for event categories.

Let me know if you have any questions!

[stephenharris]

Thanks @r-a-y - I only added the check for tags because it's possible (via the plug-in settings) that event-tag doesn't exist as a taxonomy. Didn't think of categories since that didn't come up.

[r-a-y]

Thanks for merging!