fix quiz moderate refresh with no submissions

closes CNVS-13241

fix error where quiz moderate refresh button fails when no existing quiz
submissions listed.

test plan:
  - create and publish a quiz
  - don't have any submissions on it
  - visit quiz moderate as instructor
  - hit the refresh icon in top right of moderate table
    - no errors should happen
  - as student, take and submit quiz
  - as instructor hit the refresh icon once quiz is submitted
    - student row in table should be updated

Change-Id: Ifeb97a4b4094ea50ec38c529c721aa5f7467b693
Reviewed-on: https://gerrit.instructure.com/35342
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>