fix `undefined method `pronouns' for nil:NilClass` bug

Closes: WOKE-93

Test plan:
* go to a public course as a non logged in user
* make sure the Course has a discussion topic
* load the assignment index page
* it should not have a 500 error for the XHR request

Change-Id: I45148dca86e41d3bee68b211c58a13e8aaf21b49
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/216074
Reviewed-by: Landon Gilbert-Bland <lbland@instructure.com>
Tested-by: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>

lib/api/v1/discussion_topics.rb

@@ -145,7 +145,7 @@ module Api::V1::DiscussionTopics
       end
     end
 
-    if user.pronouns
+    if user&.pronouns
       json[:user_pronouns] = user.pronouns
     end
 

spec/apis/v1/discussion_topics_api_spec.rb

@@ -144,6 +144,11 @@ describe Api::V1::DiscussionTopics do
     expect(data[:assignment]).to be_nil
   end
 
+  it "should not die if user is nil (like when a non-logged-in user visits a public course)" do
+    data = @test_api.discussion_topic_api_json(@topic, @topic.context, nil, nil)
+    expect(data).to be_present
+  end
+
   context "with assignment" do
     before :once do
       @topic.assignment = assignment_model(:course => @course)