clear discussion availability dates with null assignment dates

test plan:
* save a discussion topic with availability dates set
* edit it to set it to a graded topic but leave the
 assignment dates blank
* the original availability dates for the topic should
 not persist on the discussions index

closes #CNVS-38362

Change-Id: I28c565cb23c7fa2da3b16449ae189f44040b88c1
Reviewed-on: https://gerrit.instructure.com/123990
Tested-by: Jenkins
Reviewed-by: Dan Minkevitch <dan@instructure.com>
QA-Review: Deepeeca Soundarrajan <dsoundarrajan@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>

app/controllers/discussion_topics_controller.rb

@@ -1040,8 +1040,8 @@ class DiscussionTopicsController < ApplicationController
 
   def prefer_assignment_availability_dates(discussion_topic_hash)
     return unless params[:assignment]
-    discussion_topic_hash['delayed_post_at'] = nil if params[:assignment][:unlock_at].present?
-    discussion_topic_hash['lock_at'] = nil if params[:assignment][:lock_at].present?
+    discussion_topic_hash['delayed_post_at'] = nil if params[:assignment].has_key?(:unlock_at)
+    discussion_topic_hash['lock_at'] = nil if params[:assignment].has_key?(:lock_at)
   end
 
   # Internal: detetermines if the delayed_post_at or lock_at dates were changed

spec/apis/v1/discussion_topics_api_spec.rb

@@ -804,6 +804,16 @@ describe DiscussionTopicsController, type: :request do
         expect(@topic).not_to be_locked
         expect(@topic.delayed_post_at).to be_nil
         expect(@topic.lock_at).to be_nil
+
+        # should work even if the assignment dates are nil
+        api_call(:put, "/api/v1/courses/#{@course.id}/discussion_topics/#{@topic.id}",
+          {:controller => "discussion_topics", :action => "update", :format => "json", :course_id => @course.to_param, :topic_id => @topic.to_param},
+          {:delayed_post_at => 2.weeks.ago.as_json, :lock_at => 1.week.ago.as_json,
+            :assignment => {:unlock_at => nil, :lock_at => nil}})
+        expect(@topic.reload.assignment.reload.unlock_at).to be_nil
+        expect(@topic.assignment.lock_at).to be_nil
+        expect(@topic.delayed_post_at).to be_nil
+        expect(@topic.lock_at).to be_nil
       end
 
       it "should update due dates with cache enabled" do