Fix category slug used when doing cat Tax Query with ID

[oscarssanchez]

Description of the Change

Fixes #2242

As WordPress core sets the category_name parameter when performing a Tax Query if you are doing an id's search, this conflicts with this check EP does here: https://github.com/10up/ElasticPress/blob/develop/includes/classes/Indexable/Post/Post.php#L1003

This problem only applies to the category taxonomy.

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Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

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[Rahmon]

LGTM