only set assignment quizzes to unpublished if assignment is missing

Change-Id: Ifd829b9a7975d2cdda7f74d1e677a842aeff54ef
Reviewed-on: https://gerrit.instructure.com/43222
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>