T6917, PhabricatorApplicationTransactionNoEffectException when saving "blocking tasks" without changing them

Summary: Fixes T6917, swallow exception when saving blocking tasks with no changes

Test Plan: Open task, "Edit Blocking Tasks", save without changing, dialog should close with no exception

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6917

Differential Revision: https://secure.phabricator.com/D11353