minor #30869 [Validator] Define null return type for Constraint::getDefaultOption() (jaikdean)

This PR was submitted for the 4.2 branch but it was merged into the 3.4 branch instead (closes #30869).

Discussion
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[Validator] Define null return type for Constraint::getDefaultOption()

| Q             | A
| ------------- | ---
| Branch?       | 4.2
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

The `Constraint::getDefaultOption()` method is documented with a return type of `string`, but has no `return` statement. This behaviour is expected, as the return value is compared against `null` elsewhere in the code, but the docblock causes problems with static analysis when extending this class.

This PR corrects the documented return type to `string|null` and adds an explicit `return null`.

Commits
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03987f2ba5 Define null return type for Constraint::getDefaultOption()

src/Symfony/Component/Validator/Constraint.php

@@ -232,12 +232,13 @@ abstract class Constraint
      *
      * Override this method to define a default option.
      *
-     * @return string
+     * @return string|null
      *
      * @see __construct()
      */
     public function getDefaultOption()
     {
+        return null;
     }
 
     /**