Ember Quiz Stats - Blank state

Makes it so that if you visit the ember stats page with 0 submissions
for a quiz, the stats do not render and instead you get a nice welcoming
note that says no students have yet taken this quiz.

Closes CNVS-13454

TEST PLAN
---- ----

  - create a quiz, and another quiz
  - don't take the first, take the second:
  - go to ember quiz stats
    - visit the first quiz:
      - verify you see the new blank state
    - visit the other quiz:
      - verify you don't
  - optionally, take the first quiz and verify that you no longer see
    the blank state

Change-Id: I39fd7428cc5c72e322040b20bd5c52ffd28d9fc3
Reviewed-on: https://gerrit.instructure.com/35953
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>