don't return commentless submissions to submission comment inbox
don't return commentless submissions to submission comment inbox

fixes CNVS-16959

test plan
- as a student, make an assignment submission with no comments
- as a teacher, grade the submission without making a comment
- as both users, view the submission comment inbox
- ensure that the page loads without error

Change-Id: Id426c7c2010fccc5219938e532983dfdbd43d103
Reviewed-on: https://gerrit.instructure.com/44571
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Brad Horrocks <7b5c5d1f2eba6c540b8e53f17d87a9bd7dbf604b@instructure.com>
QA-Review: Steven Shepherd <e794458448443812fb2685675aab3bbd1a3891bc@instructure.com>
Product-Review: Janelle Seegmiller <980df7c906a86e4a61a4c3325f0e167242ce9bb0@instructure.com>

sfu/canvas-lms,redconfetti/canvas-lms,sfu/canvas-lms,roxolan/canvas-lms,kyroskoh/canvas-lms,grahamb/canvas-lms,greyhwndz/canvas-lms,antoniuslin/canvas-lms,fronteerio/canvas-lms,leftplusrightllc/canvas-lms,antoniuslin/canvas-lms,leftplusrightllc/canvas-lms,roxolan/canvas-lms,ShreniiNepal/shrenii_lmp,wrdsb/canvas-lms,skmezanul/canvas-lms,venturehive/canvas-lms,venturehive/canvas-lms,greyhwndz/canvas-lms,efrj/canvas-lms,jay3126/canvas-lms,HotChalk/canvas-lms,shidao-fm/canvas-lms,Jyaasa/canvas-lms,greyhwndz/canvas-lms,Rvor/canvas-lms,tgroshon/canvas-lms,utkarshx/canvas-lms,fronteerio/canvas-lms,whalejasmine/canvas-lms,va7map/canvas-lms,venturehive/canvas-lms,efrj/canvas-lms,kyroskoh/canvas-lms,sdeleon28/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,HotChalk/canvas-lms,jay3126/canvas-lms,utkarshx/canvas-lms,whalejasmine/canvas-lms,sdeleon28/canvas-lms,juoni/canvas-lms,kyroskoh/canvas-lms,tgroshon/canvas-lms,wrdsb/canvas-lms,grahamb/canvas-lms,djbender/canvas-lms,redconfetti/canvas-lms,Rvor/canvas-lms,skmezanul/canvas-lms,antoniuslin/canvas-lms,juoni/canvas-lms,utkarshx/canvas-lms,grahamb/canvas-lms,Jyaasa/canvas-lms,shidao-fm/canvas-lms,Rvor/canvas-lms,efrj/canvas-lms,grahamb/canvas-lms,efrj/canvas-lms,shidao-fm/canvas-lms,SwinburneOnline/canvas-lms,Jyaasa/canvas-lms,ShreniiNepal/shrenii_lmp,sdeleon28/canvas-lms,leftplusrightllc/canvas-lms,redconfetti/canvas-lms,Jyaasa/canvas-lms,va7map/canvas-lms,skmezanul/canvas-lms,djbender/canvas-lms,greyhwndz/canvas-lms,utkarshx/canvas-lms,shidao-fm/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,va7map/canvas-lms,samuels410/greatlakes-lms,roxolan/canvas-lms,grahamb/canvas-lms,ShreniiNepal/shrenii_lmp,samuels410/greatlakes-lms,redconfetti/canvas-lms,jay3126/canvas-lms,roxolan/canvas-lms,samuels410/greatlakes-lms,venturehive/canvas-lms,fronteerio/canvas-lms,sfu/canvas-lms,HotChalk/canvas-lms,djbender/canvas-lms,Rvor/canvas-lms,kyroskoh/canvas-lms,tgroshon/canvas-lms,HotChalk/canvas-lms,leftplusrightllc/canvas-lms,ShreniiNepal/shrenii_lmp,fronteerio/canvas-lms,juoni/canvas-lms,SwinburneOnline/canvas-lms,SwinburneOnline/canvas-lms,juoni/canvas-lms,antoniuslin/canvas-lms,whalejasmine/canvas-lms,sdeleon28/canvas-lms,wrdsb/canvas-lms,wrdsb/canvas-lms,SwinburneOnline/canvas-lms,jay3126/canvas-lms,skmezanul/canvas-lms,tgroshon/canvas-lms,whalejasmine/canvas-lms,va7map/canvas-lms,samuels410/greatlakes-lms