minor #33257 [Security/Core] UserInterface::getPassword() can return null (nicolas-grekas)

This PR was merged into the 3.4 branch.

Discussion
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[Security/Core] UserInterface::getPassword() can return null

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Our very own `User` class can return null already.

Commits
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00d7f8cde7 [Security/Core] UserInterface::getPassword() can return null

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

@@ -55,7 +55,7 @@ interface UserInterface
      * This should be the encoded password. On authentication, a plain-text
      * password will be salted, encoded, and then compared to this value.
      *
-     * @return string The password
+     * @return string|null The encoded password if any
      */
     public function getPassword();