Fix post_parent returning early if set to 0

[felipeelia]

This is a simple takeover of #3196

Description of the Change

This change changes empty( $args['post_parent'] ) to ! isset( $args['post_parent'] ) so any query or pre_get_posts (or similar) filter that sets post_parent to 0 will still work as expected.

Closes #3181

How to test the Change

As seen in #3181, use a filter like the following on a hierarchical CPT to confirm the confirm child posts are actually removed.

add_action( 'pre_get_posts', function( $query ) {
	if ( is_admin() ) {
		return;
	}

	if ( ! $query->is_main_query() ) {
		return;
	}

	if ( ! $query->is_post_type_archive( 'things' ) ) {
		return;
	}

	$query->set( 'post_parent', 0 );
});

Changelog Entry

Fixed - Queries with post_parent set to 0 not working correctly

Credits

Props @JiveDig

Checklist:

• I agree to follow this project's Code of Conduct.
• [] I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.