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