missing policy: ignore submissions for unpublished assignments

Changes the Missing Policy so that it does not apply grades to
missing submissions for unpublished assignments.

closes TALLY-610
flag=none

Test Plan:
* have a course with a student enrolled
* create a missing policy for the course
* create an assignment with a due and until date for the
  near future and do not publish the assignment
* wait for the due date to pass, and for the missing policy to run
* verify no submissions for the assignment were graded

Change-Id: I81c46154c2735d467cafb174aab44321115833bb
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/227591
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Adrian Packel <apackel@instructure.com>
Reviewed-by: Jeremy Neander <jneander@instructure.com>
QA-Review: Adrian Packel <apackel@instructure.com>
QA-Review: Robin Kuss <rkuss@instructure.com>
Product-Review: Jonathan Fenton <jfenton@instructure.com>

lib/missing_policy_applicator.rb

@@ -37,6 +37,7 @@ class MissingPolicyApplicator
       joins(assignment: {course: :late_policy}).
       eager_load(:grading_period, assignment: [:post_policy, { course: [:late_policy, :default_post_policy] }]).
       for_enrollments(Enrollment.all_active_or_pending).
+      merge(Assignment.published).
       missing.
       where(score: nil, grade: nil, cached_due_date: 1.day.ago(now)..now,
             late_policies: { missing_submission_deduction_enabled: true })

spec/lib/missing_policy_applicator_spec.rb

@@ -121,6 +121,18 @@ describe MissingPolicyApplicator do
       expect(submission.workflow_state).to eql 'graded'
     end
 
+    it 'ignores submissions for unpublished assignments' do
+      assignment = create_recent_assignment
+      assignment.unpublish
+      late_policy_missing_enabled
+      applicator.apply_missing_deductions
+
+      submission = @course.submissions.first
+
+      expect(submission.score).to be_nil
+      expect(submission.grade).to be_nil
+    end
+
     context "updated timestamps" do
       before(:once) do
         @frozen_now = now