restore quiz submission loading in quiz serializer

This reverts changes from 8018ce8374e9364170e639952909cd9fc588287a.

Instead of using `present?` on has_many associations for
Canvas::APISerializer, use `empty?` so that we don't overload the
database or app memory with large queries for big data sets.

Test Plan:
  - Create a quiz.
  - In your console, run:
    quiz = Quizzes::Quiz.find(your_quiz_id)
    10_000.times { quiz.quiz_submissions.create! }
  - Get a cup or two of coffee and wait for that to finish.
  - You should be able to load the quiz show page for that quiz.
  - Making an API for that request as the teacher should not time out.

closes CNVS-12098

Change-Id: Ib14bc30583f521bd91a3de115218a33fb3bd73c4
Reviewed-on: https://gerrit.instructure.com/32553
Reviewed-by: Brian Palmer <brianp@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Stanley Stuart <stanley@instructure.com>