Warnings due to MetatagFirehose

#2563639: Global configuration via config entities added the function metatag_get_default_tags(), and modified MetatagFirehose::formElement to use the default tags in a foreach. The problem is that metatag_get_default_tags() can return null and the foreach doesn't check for a null value first, resulting in this warning anywhere a metatags field appears (such as a node edit form):

Warning: Invalid argument supplied for foreach() in Drupal\metatag\Plugin\Field\FieldWidget\MetatagFirehose->formElement() (line 71 of modules/contrib/metatag/src/Plugin/Field/FieldWidget/MetatagFirehose.php)

Patch to follow.