Taxonomy: Fix cache issue in get_the_terms

[dingo-d]

Loosened the condition to check the terms fetched fro mthe object term cache. The cache can return an empty array, which is a falsy value, so the get_the_terms will return false value.

Trac ticket: https://core.trac.wordpress.org/ticket/49799

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[dingo-d]

Also, maybe a good thing to note is that I'm assigning the terms in my tests using wp_set_object_terms(); function. The same thing happened when I tried wp_set_post_terms(), so it could be something related to that (test env could mess with the object caching?).

I'm wondering: in the case there is nothing in the post (no assigned term), the

$term_ids = wp_list_pluck( $terms, 'term_id' );
wp_cache_add( $post->ID, $term_ids, $taxonomy . '_relationships' );

will write empty array to the cache. Is this ok or is it redundant?

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