minor #30119 [Form] add missing null return type (xabbuh)

This PR was merged into the 3.4 branch.

Discussion
----------

[Form] add missing null return type

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

This reflects how all our implementations of this interface actually behave.

Commits
-------

03abfc09bd add missing null return type

Author: xabbuh

FormTypeGuesserInterface.php

@@ -32,7 +32,7 @@ public function guessType($class, $property);
      * @param string $class    The fully qualified class name
      * @param string $property The name of the property to guess for
      *
-     * @return Guess\ValueGuess A guess for the field's required setting
+     * @return Guess\ValueGuess|null A guess for the field's required setting
      */
     public function guessRequired($class, $property);