Fix media check on default domain

[ekes]

When the microsites controller tries to add a node on the default domain (in a group or for the control site itself):-

The website encountered an unexpected error. Please try again later.
AssertionError: Cannot load the "group" entity with NULL ID. in assert() (line 295 of core/lib/Drupal/Core/Entity/EntityStorageBase.php).

assert(, 'Cannot load the "group" entity with NULL ID.') (Line: 295)
Drupal\Core\Entity\EntityStorageBase->load(NULL) (Line: 19)
localgov_microsites_media_node_create_access(Object, Array, 'localgov_page')

Patch adds a check for group_id before loading it. Also does an attempt to pull from the path as alternative to domain as addition - would only be needed if a site admin was on the control site, but it was a quick easy add for that occasion (or if the controller is masquerading as them to do something on the control domain)

[finnlewis]

Confirm that I get the error without this change and that the error goes away with this branch.