Fix undefined variable in HeraldAction

Summary: $no_permission is only defined within if ($object instanceof PhabricatorPolicyInterface), thus move the check for if ($no_permission) into that clause.

Test Plan: Read the code.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25741