prevent NoMethodErrors when generating quiz statistics

Test plan:
  - Create a quiz with an essay question.
  - As a student, take the quiz but leave the question blank.
  - As the teacher, you should be able to generate item analysis
    and student reports without an error page.

refs CNVS-10773

Change-Id: Id8670751fb10a1faf89f1d3377a6426cf4b25d17
Reviewed-on: https://gerrit.instructure.com/29891
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Myller de Araujo <myller@instructure.com>
Product-Review: Stanley Stuart <stanley@instructure.com>