submission missing for submission types and scores of zero

refs: CNVS-10301

This is the canvas-lms part of the fix for the ticket.  This adds logic to
the submission model missing? method to take into consideration whether the
assignment expects a submission or not and has been graded with a grade of zero.

Change-Id: I043dab161e1765268e34464ec49fd0276b16acbb
Reviewed-on: https://gerrit.instructure.com/29186
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Product-Review: Nick Cloward <ncloward@instructure.com>
QA-Review: Nick Cloward <ncloward@instructure.com>
Tested-by: Nick Cloward <ncloward@instructure.com>