Disallow awarding a badge without selecting recipient

Summary:
Do not continue on missing fields (in this case: the badge recipient) in PhabricatorBadgesEditRecipientsController.

Closes T15827

Test Plan: See steps in T15827.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15827

Differential Revision: https://we.phorge.it/D25740