Numerical question answer of 0 should be displayed

Closes CNVS-13831

TEST PLAN
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  - edit a quiz
  - create a numerical question
  - choose an "Exact answer" with a number of 0
  - click Update Question
    + re-edit the question
    + verify you see 0 as the answer, not an empty input box
  - rinse and repeat with other numbers, and back to 0

Change-Id: Ic6c45665519055d37e9aa480ce1b09ea48d4d1f3
Reviewed-on: https://gerrit.instructure.com/41319
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Taylor <rtaylor@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>