Curb attempt to iterate over NULL from getParentNidsOfFileEntity()

[qadan]

\Drupal\embargoes\EmbargoesEmbargoesService\getParentNidsOfFileEntity() returns either NULL or an array depending on found relationships. Both times it's being used in practice in embargoes.module, the results are being passed to getActiveEmbargoesByNids(), which passes them through to getCurrentEmbargoesByNids(), which passes them through to getAllEmbargoesByNids(), which attempts to iterate over them as an array. If the file entity doesn't have any parents, iterating over NULL causes an error to be thrown.

It's hard to tell the direction to go in here (could do a null check in the .module), but I'm thinking it's perhaps reasonable to enforce that getParentNidsOfFileEntity() returns an array that may be empty if there are no such node IDs or aren't accessible.

[qadan]

Superseded by #15