minor #10717 unified return null usages (fabpot)

This PR was merged into the 2.3 branch.

Discussion
----------

unified return null usages

| Q             | A
| ------------- | ---
| License       | MIT

This PR unifies the way we return `null` from a function or method:

 * always use `return;` instead of `return null;` (the current code base uses both);
 * never use `return;` at the end of a function/method.

Commits
-------

d1d569b unified return null usages

Author: fabpot

Authentication/Provider/AnonymousAuthenticationProvider.php

@@ -40,7 +40,7 @@ public function __construct($key)
     public function authenticate(TokenInterface $token)
     {
         if (!$this->supports($token)) {
-            return null;
+            return;
         }
 
         if ($this->key !== $token->getKey()) {

Authentication/Provider/PreAuthenticatedAuthenticationProvider.php

@@ -53,7 +53,7 @@ public function __construct(UserProviderInterface $userProvider, UserCheckerInte
      public function authenticate(TokenInterface $token)
      {
          if (!$this->supports($token)) {
-             return null;
+             return;
          }
 
         if (!$user = $token->getUser()) {

Authentication/Provider/UserAuthenticationProvider.php

@@ -58,7 +58,7 @@ public function __construct(UserCheckerInterface $userChecker, $providerKey, $hi
     public function authenticate(TokenInterface $token)
     {
         if (!$this->supports($token)) {
-            return null;
+            return;
         }
 
         $username = $token->getUsername();

User/User.php

@@ -64,7 +64,6 @@ public function getPassword()
      */
     public function getSalt()
     {
-        return null;
     }
 
     /**