fix failing user model specs

bare_submission_model doesn't validate (#valid? returns nil!?!?) as
used here

Change-Id: I2623274156ea99c30bb67f7fc03be2757d1f1ae2
Reviewed-on: https://gerrit.instructure.com/30082
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>
QA-Review: Cameron Matheson <cameron@instructure.com>