userHelpers tests: Use NULL_USER.avatar_url.

This will mean one less thing we have to change when we start using
the new format for `avatar_url` in an upcoming commit.

src/users/__tests__/userHelpers-test.js

@@ -14,6 +14,7 @@ import {
   groupUsersByStatus,
 } from '../userHelpers';
 import * as eg from '../../__tests__/lib/exampleData';
+import { NULL_USER } from '../../nullObjects';
 
 describe('filterUserList', () => {
   test('empty input results in empty list', () => {
@@ -74,7 +75,7 @@ describe('getAutocompleteSuggestion', () => {
         full_name: 'all',
         email: '(Notify everyone)',
         user_id: -1,
-        avatar_url: '',
+        avatar_url: NULL_USER.avatar_url,
         timezone: '',
         is_admin: false,
         is_bot: false,