Taxonomy: Check for empty term after DB sanitization in wp_insert_term

[fgiannar]

When inserting a new term in the database, wp_insert_term will check if the term is empty and return a corresponding error.
Afterwards the term is sanitized and inserted in the database. However, there's a chance the term is empty after the DB sanitization.
This is never checked though and will result in 1) empty terms in the DB and 2) possible OOM issues in wp_insert_term when checking if the term name matches an existing one because of using get_terms with an empty name argument

The current PR:

• Adds a check for empty term name after the term is sanitized and returns an error if the term is empty.
• Adds a corresponding unit test

Trac ticket: https://core.trac.wordpress.org/ticket/59995

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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[fgiannar]

Typo

[kraftbj]

This tests well for me with a dirty term name. Hotfixing in a production environment also resulted in expected change without any known regressions at this time. Unit test appears sufficient along side existing tests checking valid term names.

[SergeyBiryukov]

Thanks for the PR! Merged in r57251.