minor #32144 add missing return type in User class (Tobion)

This PR was merged into the 5.0-dev branch.

Discussion
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add missing return type in User class

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | #...   <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!-- required for new features -->

Fixed a missing type in #31996 chasing #17201
Every method in this class had a return type but this method.

Commits
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7857f2f add missing return type in User class

src/Symfony/Component/Security/Core/User/User.php

@@ -67,8 +67,9 @@ public function getPassword(): ?string
     /**
      * {@inheritdoc}
      */
-    public function getSalt()
+    public function getSalt(): ?string
     {
+        return null;
     }
 
     /**