Prevents funky errors when accessing quiz_data#compact

Closes CNVS-21228, GH-291, GH-512

Test Plan:
- Quizzes still work

Change-Id: I1b2131a8e0c8bd3e015597f0f756394217ee16b3
Reviewed-on: https://gerrit.instructure.com/56469
Tested-by: Jenkins
Reviewed-by: Brian Finney <bfinney@instructure.com>
QA-Review: Michael Hargiss <mhargiss@instructure.com>
Product-Review: Ryan Taylor <rtaylor@instructure.com>

app/models/quizzes/quiz_submission.rb

@@ -372,7 +372,7 @@ class Quizzes::QuizSubmission < ActiveRecord::Base
   end
 
   def questions_as_object
-    self.quiz_data || {}
+    self.quiz_data || []
   end
 
   def quiz_question_ids