Prevent warnings in get_the_category from unregistered "category" taxonomy

[batmoo]

get_the_terms can return a WP_Error object if the category taxonomy is not defined (something that sites can do if they want custom taxonomies). This results in some warnings from get_the_category because it assumes that the category taxonomy will always be there and it will always get a valid value back.

Warning: array_values() expects parameter 1 to be array, object given in /home/wpcom/public_html/wp-includes/category-template.php on line 79

Warning: array_keys() expects parameter 1 to be array, null given in /home/wpcom/public_html/wp-includes/category-template.php on line 81

Warning: Invalid argument supplied for foreach() in /home/wpcom/public_html/wp-includes/category-template.php on line 81

Attached patches adds an is_wp_error check to prevent these warnings.

Related: #19590

In 22412:

Check for WP_Error in get_the_category(). props batmoo. fixes #21155.