minor symfony#16635 [Security] add USERNAME_NONE_PROVIDED constant (SofHad)

This PR was submitted for the 2.8 branch but it was merged into the 3.1-dev branch instead (closes symfony#16635).

Discussion
----------

[Security] add USERNAME_NONE_PROVIDED constant

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | no
| License       | MIT
| Doc PR        | no

I think that is better if we use a constant to define this string.

Commits
-------

3f60598 [Security] add USERNAME_NONE_PROVIDED constant

Author: fabpot

src/Symfony/Component/Security/Core/Authentication/Provider/AuthenticationProviderInterface.php

@@ -24,6 +24,13 @@
  */
 interface AuthenticationProviderInterface extends AuthenticationManagerInterface
 {
+    /**
+     * Use this constant for not provided username
+     *
+     * @var string
+     */
+    const USERNAME_NONE_PROVIDED = 'NONE_PROVIDED';
+
     /**
      * Checks whether this provider supports the given token.
      *

src/Symfony/Component/Security/Core/Authentication/Provider/LdapBindAuthenticationProvider.php

@@ -58,7 +58,7 @@ public function __construct(UserProviderInterface $userProvider, UserCheckerInte
      */
     protected function retrieveUser($username, UsernamePasswordToken $token)
     {
-        if ('NONE_PROVIDED' === $username) {
+        if (AuthenticationProviderInterface::USERNAME_NONE_PROVIDED === $username) {
             throw new UsernameNotFoundException('Username can not be null');
         }
 

src/Symfony/Component/Security/Core/Authentication/Provider/UserAuthenticationProvider.php

@@ -63,7 +63,7 @@ public function authenticate(TokenInterface $token)
 
         $username = $token->getUsername();
         if ('' === $username || null === $username) {
-            $username = 'NONE_PROVIDED';
+            $username = AuthenticationProviderInterface::USERNAME_NONE_PROVIDED;
         }
 
         try {