Performance: Use get_term() instead of term_exists() for get_term_tree()

[rebeccahum]

Description of the Change

This is a follow-up to the change in #2687. It'd be better to use get_term() to utilize the Object cache, since term_exists() does a DB call and thus, if there are several terms with the same parent, that generates a lot of queries.

Alternate Designs

N/A.

Possible Drawbacks

N/A.

Verification Process

Did the same steps described in #2687 but ensure that object caching is utilized.

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.

Changelog Entry

Changed: Improved performance in get_term_tree()

Credits

Props @rebeccahum

[felipeelia]

@rebeccahum looking at the get_term() documentation, it can also return null for some specific failures, so we'll probably have to test for that in addition to the is_wp_error() you already have there.

[rebeccahum]

@felipeelia I've updated it to account for a null return.