use null dates if not specified in all overrides

- matches logic in assignment_override_applicator.rb

closes QO-387

Test plan:
- create quiz with overrides
  - Everyone and section overrides with lock_at
    in the past
  - individual student override with lock_at
    not specified
- view quizzes index page as student with override
  above
- verify that quiz does not say "Closed"

Change-Id: Ie70acee7429e75bfa7d1b89017d9be70225a8111
Reviewed-on: https://gerrit.instructure.com/169876
Tested-by: Jenkins
Reviewed-by: Keith Garner <kgarner@instructure.com>
Reviewed-by: Jacob Burroughs <jburroughs@instructure.com>
QA-Review: Jacob Burroughs <jburroughs@instructure.com>
Product-Review: Kevin Dougherty III <jdougherty@instructure.com>

app/coffeescripts/models/QuizOverrideLoader.coffee

@@ -44,11 +44,15 @@ define [
       quiz.set('loadingOverrides', false)
 
     _chooseLatest: (dates, type) ->
+      if _.any(dates, (d) -> _.isNull(d[type]) || _.isUndefined(d[type]))
+        return null
       sortedDates = @_sortedDatesOfType(dates, type)
       if _.any(sortedDates)
         _.last(sortedDates)
 
     _chooseEarliest: (dates, type) ->
+      if _.any(dates, (d) -> _.isNull(d[type]) || _.isUndefined(d[type]))
+        return null
       sortedDates = @_sortedDatesOfType(dates, type)
       if _.any(sortedDates)
         _.first(sortedDates)

spec/coffeescripts/quizzes/QuizOverrideLoaderSpec.js

@@ -41,9 +41,15 @@ test('can select the earliest date from a group', function() {
   equal(this.loader._chooseEarliest(this.dates, 'unlock_at'), this.earliestDate)
 })
 
-test('ignores null dates and handles empty arrays', function() {
+test('handles null dates and handles empty arrays', function() {
   let dates = [{}, {}]
   equal(this.loader._chooseLatest(dates, 'due_at'), null)
   dates = []
   equal(this.loader._chooseLatest(dates, 'due_at'), null)
 })
+
+test('returns null if any argument is null', function() {
+  const dates = [...this.dates, {}]
+  equal(this.loader._chooseLatest(dates, 'due_at'), null)
+  equal(this.loader._chooseEarliest(dates, 'due_at'), null)
+})