bug #5149 Fixed loadUserByUsername method coding errors (Linas Merkevicius)

This PR was submitted for the 2.6 branch but it was merged into the 2.3 branch instead (closes #5149).

Discussion
----------

Fixed loadUserByUsername method coding errors

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.6, 2.7, 3.0
| Fixed tickets |

Commits
-------

3a7acfb Fixed loadUserByUsername function coding errors

Author: wouterj

cookbook/security/entity_provider.rst

@@ -435,9 +435,9 @@ interface requires three methods: ``loadUserByUsername($username)``,
                 ->setParameter('username', $username)
                 ->setParameter('email', $username)
                 ->getQuery()
-                ->getOneOrNullResult()
+                ->getOneOrNullResult();
 
-            if ($user) {
+            if (null === $user) {
                 $message = sprintf(
                     'Unable to find an active admin AppBundle:User object identified by "%s".',
                     $username