CORE-842 Prevent script error in join model reinit()

If a join-keyed value is missing from a join model instance, reinit() would fail to work correctly.  We were seeing this cause errors on delete after the BackgroundTask returned and caused some objects to be stub_destroyed and destroyed. I.e. "section" key values were missing from SectionObjective after the SectionObjective had been destroyed on the server and was being cleaned up on the client.  This is a simple guard against throwing errors in that case.