Ember Quiz Stats - Discrimination Index Sorting

Fixes an issue where sorting by the DIndex would position questions with
negative indices below questions that don't have a DI at all.

Closes CNVS-13384

TEST PLAN
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  - create a quiz with a few questions of type multiple-choice, and
    another of a different type
  - go to ember quiz stats:
    - sort by discrimination index
      - verify that MC/TF questions are sorted below each other, and
        other questions that don't have a DIndex are below them

Change-Id: I512b7e9fe5e2c909d7145415e022ab49abc1d889
Reviewed-on: https://gerrit.instructure.com/35737
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>