get_term_tree(): fix infinite loop for non-existent parent

[rebeccahum]

Description of the Change

When calling get_term_tree() and a term has a parent ID that doesn't exist (possibly due to reasons such as a bad import), we should check for that to prevent an infinite loop from occurring.

Reproduction steps:

$terms = array(
	0 => (object)  array
	(
		'term_id' => 2,
		'name' => 'ingredients',
		'slug' => 'ingredients',
		'term_group' => 0,
		'term_taxonomy_id' => 2,
		'taxonomy' => 'category',
		'description' => '',
		'parent' => 0,
		'count' => 1,
		'filter' => 'raw'
	),

);

$terms[0]->parent = 100; // non-existing parent in the terms

print_r( \ElasticPress\Utils\get_term_tree( $terms ) );

Alternate Designs

N/A

Possible Drawbacks

None that comes to mind.

Verification Process

Reproduce the reproduction steps with the PR applied and ensure no critical errors occur.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Changelog Entry

Fixed - get_term_tree() will no longer infinite loop when parent ID is non-existent

Credits

Props @rebeccahum