fix QuizRegrading::Submission when ActiveSupport isn't around

no need to use present? to check nilness

Change-Id: If7a974d5f1be77a6657f56946a02cf8a51388a91
Reviewed-on: https://gerrit.instructure.com/29684
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>