Return valid response in Projects::MergeRequestsController#ci_status when diff_head_sha is nil

app/controllers/projects/merge_requests_controller.rb

@@ -417,7 +417,7 @@ class Projects::MergeRequestsController < Projects::ApplicationController
 
     response = {
       title: merge_request.title,
-      sha: merge_request.diff_head_commit.short_id,
+      sha: (merge_request.diff_head_commit.short_id if merge_request.diff_head_sha),
       status: status,
       coverage: coverage
     }