Add a missing test case

src/sidebar/components/test/annotation-header-test.js

@@ -81,6 +81,13 @@ describe('sidebar.components.annotation-header', function () {
           user_info: undefined,
           expectedResult: 'bill',
         },
+        {
+          context: 'when the api_render_user_info feature flag is on but ' +
+                   "the user doesn't have a display_name in h",
+          it: 'returns the username',
+          user_info: { display_name: null },
+          expectedResult: 'bill',
+        },
         {
           context: 'when the api_render_user_info feature flag is on and ' +
                    "the user has a display_name",