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 corresponing 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

Example: wp_insert_term( '<script>Yo</script>', 'post_tag' );

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

[fgiannar]

Replying to fgiannar:

Example: wp_insert_term( '<script>Yo</script>', 'post_tag' );

Example: wp_insert_term( '<script>onclick=alert("hello")</script>', 'post_tag' );

[SergeyBiryukov]

In 57251:

Taxonomy: Check for empty term after DB sanitization in wp_insert_term().

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 is a chance the term is empty after the DB sanitization.

This commit adds a check for an empty term name after the term is sanitized, returning an error in that case.

Follow-up to [5726], [8393].

Props fgiannar, kraftbj.
Fixes #59995.

Thanks for the PR! Merged in r57251.