Fix PhabricatorEmptyQueryException handling in DiffusionCommitQuery::loadPage()
Summary: Add an additional check that `$empty_exception` is not `null` before try to `throw` it.
Test Plan: Read the previous code; run static code analysis.
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/D25781