don't show user avatar path in anonymous peer review api

test plan:
* enable avatars on the root account
* create an assignment with anonymous peer reviews
* as a peer, make a review and a comment on a submission
* as the reviewed student, visit the api index for
 peer reviews, e.g.
 "api/v1/courses/X/assignments/Y/peer_reviews?
  include[]=submission_comments&include[]=user"
* it should not include the user id in the avatar_path

closes #CNVS-24046

Change-Id: I029aa9ebb203d43f546a1bb8e8f7b29160ebdba7
Reviewed-on: https://gerrit.instructure.com/65445
Tested-by: Jenkins
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>

lib/api/v1/submission_comment.rb

@@ -37,6 +37,9 @@ module Api::V1::SubmissionComment
     if submission_comment.grants_right?(@current_user, :read_author)
       sc_hash['author'] = user_display_json(submission_comment.author, submission_comment.context)
     else
+      if sc_hash.delete('avatar_path')
+        sc_hash['avatar_path'] = User.default_avatar_fallback
+      end
       sc_hash.merge!({
                       author: {},
                       author_id: nil,

spec/apis/v1/peer_reviews_api_spec.rb

@@ -383,6 +383,7 @@ describe PeerReviewsApiController, type: :request do
 
 
       it 'should not include submission comments user information when anonymous peer reviews' do
+        @course.root_account.tap{|a| a.enable_service(:avatars)}.save!
         @assignment1.update_attribute(:anonymous_peer_reviews, true)
         @comment = @submission.add_comment(:author => @student2, :comment => "review comment")
         @user = @student1
@@ -393,6 +394,7 @@ describe PeerReviewsApiController, type: :request do
                                "id"=>@assessment_request.id,
                                "submission_comments" => [{"author_id"=>nil,
                                                           "author_name"=>"Anonymous User",
+                                                          'avatar_path' => User.default_avatar_fallback,
                                                           "comment"=>"review comment",
                                                           "created_at"=>@comment.created_at.as_json,
                                                           "id"=>@comment.id,