minor #49896 Apply no_null_property_initialization PHP-CS-Fixer rule (yguedidi)

This PR was merged into the 6.3 branch.

Discussion
----------

Apply no_null_property_initialization PHP-CS-Fixer rule

| Q             | A
| ------------- | ---
| Branch?       | 6.3
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       |
| License       | MIT
| Doc PR        |

After a discussion with `@stof`, this rule should be part of the ``@Symfony`` PHP-CS-Fixer ruleset.
This PR apply the rule on the Symfony code base.
see PHP-CS-Fixer/PHP-CS-Fixer#6876 for the PR on PHP-CS-Fixer

Commits
-------

2320c2a906 Apply no_null_property_initialization PHP-CS-Fixer rule

Author: nicolas-grekas

Tests/RouterTest.php

@@ -25,9 +25,9 @@
 
 class RouterTest extends TestCase
 {
-    private $router = null;
+    private $router;
 
-    private $loader = null;
+    private $loader;
 
     private $cacheDir;