minor #22418 [HttpFoundation] Fix getClientIp @return docblock (e-moe)

This PR was merged into the 2.7 branch.

Discussion
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[HttpFoundation] Fix getClientIp @return docblock

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #22415
| License       | MIT
| Doc PR        | none

Commits
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af3424f [HttpFoundation] Fix getClientIp @return docblock

src/Symfony/Component/HttpFoundation/Request.php

@@ -850,7 +850,7 @@ public function getClientIps()
      * ("Client-Ip" for instance), configure it via "setTrustedHeaderName()" with
      * the "client-ip" key.
      *
-     * @return string The client IP address
+     * @return string|null The client IP address
      *
      * @see getClientIps()
      * @see http://en.wikipedia.org/wiki/X-Forwarded-For