Changeset 36988 for trunk/tests/phpunit/tests/category/getCategories.php

Improve error handling in get_categories().

When passed an invalid 'taxonomy', get_terms() will return a WP_Error
object. This object should not be blindly cast to an array. Instead, an empty
array should be returned, to indicate that no matching terms have been found.

Props virgodesign, sebastian.pisula.
Fixes #36227.