REST API: Undefined-property notice when setting term parent to 0

[dlh]

Attempting to create or update a term with a parent of 0 using the REST API generates a undefined-property notice because a WP_Error object is treated as a WP_Term.

The WP_Error comes from calling get_term( 0 ), and the terms controller checks for only a truthy response before attempting to access the term_id property.

The attached patch includes tests demonstrating the issue, adds verification that the response from get_term() is a term, and, as a tiny optimization, skips the get_term() check when the provided parent is 0.

[earnjam]

@dlh One small thing I noticed in the patch.

This part is redundant, because $parent_term_id is already initialized as 0 right above this on line 672.

<?php
if ( 0 === $requested_parent ) {
    $parent_term_id = $requested_parent;
}

[dlh]

Yeah, it's funny-looking. The only reason it's there is so that the logic in that condition would be isolated from a change in the initialization, should that ever happen. No big deal to me either way, though.

[desrosj]

Since this is only a notice, let's bump it to 5.1.

[pento]

Patch needs review and a decision.

[kadamwhite]

Adjust patch based on feedback from @earnjam and fix PHPCS spacing issue

[kadamwhite]

In 44965:

REST API: Avoid undefined-property notice when setting parent term to 0.

Only try to access term_id once $parent_term is known to be a WP_Term.

Props dlh, earnjam.
Fixes #44983.

[TimothyBlynJacobs]

#43772 was marked as a duplicate.