Fix: Term search in admin panel for non-public taxonomies

[burhandodhy]

Description of the Change

This PR fixes the issue where the search term in the admin panel returns nothing for the non-public taxonomies. The root cause of the problem is that ElasticPress doesn't sync any non-public taxonomies whether the protected content feature is enabled or not. But when a user searches for the term, EP tries to get the data from ElasticSearh.

My fix added an additional check that will check if all the taxonomies in the query are indexable. If not, then EP will not try to fetch the data from ElasticSearch. 

Closes #2930

How to test the Change

1. Enable Terms Feature
2. Create new taxonomy and set the public parameter to false
3. Create a new term
4. Search for term in admin dashboard.

Changelog Entry

Fixed - Term search in the admin panel for non-public taxonomies returns nothing.

Credits

Props @burhandodhy

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.