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