fix: validateJson should return false when value is null

Return false when $value is null.

Avoid TypeError: json_validate(): Argument #1 ($json) must be of type string, null given, when using symfony/polyfill-php83 in PHP < 8.3.

Avoid deprecation warning: json_validate(): Passing null to parameter #1 ($json) of type string is deprecated, when using PHP 8.3.

src/Illuminate/Validation/Concerns/ValidatesAttributes.php

@@ -1430,11 +1430,11 @@ public function validateMacAddress($attribute, $value)
      */
     public function validateJson($attribute, $value)
     {
-        if (is_array($value)) {
+        if (is_array($value) || is_null($value)) {
             return false;
         }
 
-        if (! is_scalar($value) && ! is_null($value) && ! method_exists($value, '__toString')) {
+        if (! is_scalar($value) && ! method_exists($value, '__toString')) {
             return false;
         }