fix importing assignments with no name

test plan:
 * import a course package containing a nameless assignment,
   and choose to remove dates from the imported course.
   You can find such a package here:
   https://hilary.instructure.com/courses/332573/content_migrations
   (download the .imscc linked to on that page)
   Note that you can't easily create a new nameless assignment,
   because CNVS-12938 adds a validator to prevent this
   (the linked course package predates that fix)
 * the nameless assignment should have a generic name
   ("untitled assignment")
 * go to the syllabus page in the new course
 * there should be no due dates there

fixes CNVS-16624

Change-Id: Ib51773024ccaac2241619cd8b1a5a8b54b99f3ce
Reviewed-on: https://gerrit.instructure.com/43791
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>