fix Submission.bulk_load_versioned_attachments

fixes CNVS-15394

This would blow up if the submission had nonexistant attachment_ids

Test plan:
  * make a submission for an assignment
  * in the console, find your submission and set attachment_ids to
    123456789 (or something)
  * Use the "List submissions for multiple assignments" API for the
    course.  It should work.

Change-Id: Iefcf71b194ece8ffbceebe3176622ede20b909fa
Reviewed-on: https://gerrit.instructure.com/40841
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>