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

This PR was merged into the 6.3 branch.

Discussion
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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
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2320c2a906 Apply no_null_property_initialization PHP-CS-Fixer rule

Author: nicolas-grekas

Validator/Constraints/UniqueEntity.php

@@ -32,11 +32,11 @@ class UniqueEntity extends Constraint
 
     public $message = 'This value is already used.';
     public $service = 'doctrine.orm.validator.unique';
-    public $em = null;
-    public $entityClass = null;
+    public $em;
+    public $entityClass;
     public $repositoryMethod = 'findBy';
     public $fields = [];
-    public $errorPath = null;
+    public $errorPath;
     public $ignoreNull = true;
 
     /**