returns blank avatar when avatar template is given nil user

fixes CNVS-12080

test plan
- as a student, make a submission to a graded discussion topic
- add the submission to the student's eportfolio
- view the eportfolio page
- ensure that no page error is produced

Change-Id: I63d3120f6bb26d37db90d9081e889fd2757fcfcd
Reviewed-on: https://gerrit.instructure.com/32972
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <banderson@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>