display scores of zero in quiz moderate

ensure that scores with a value of zero are displayed in the moderate
table.

closes CNVS-13335

test plan:
  - create a quiz (of any type)
  - as a student
    - take the quiz, and answer all question incorrectly
  - as instructor
    - visit the quiz moderate view and verify score is displayed
  - verify that:
    - submissions with score > 0 are still displayed
    - users without an submission don't display a score

Change-Id: Ia44229a9ee792bfe4f0c3e9b4176fef932413d7f
Reviewed-on: https://gerrit.instructure.com/35570
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Jason Madsen <jmadsen@instructure.com>