test: validateJson should return false when value is null

Fails with Laravel Framework 10.38.2 in PHP < 8.3, introduced in laravel#49413

tests/Validation/ValidationValidatorTest.php

@@ -19,6 +19,7 @@
 use Illuminate\Support\Arr;
 use Illuminate\Support\Carbon;
 use Illuminate\Support\Exceptions\MathException;
+use Illuminate\Support\Stringable;
 use Illuminate\Translation\ArrayLoader;
 use Illuminate\Translation\Translator;
 use Illuminate\Validation\DatabasePresenceVerifierInterface;
@@ -2817,6 +2818,14 @@ public function testValidateJson()
         $trans = $this->getIlluminateArrayTranslator();
         $v = new Validator($trans, ['foo' => ['array']], ['foo' => 'json']);
         $this->assertFalse($v->passes());
+
+        $trans = $this->getIlluminateArrayTranslator();
+        $v = new Validator($trans, ['foo' => null], ['foo' => 'json']);
+        $this->assertFalse($v->passes());
+
+        $trans = $this->getIlluminateArrayTranslator();
+        $v = new Validator($trans, ['foo' => new Stringable('[]')], ['foo' => 'json']);
+        $this->assertTrue($v->passes());
     }
 
     public function testValidateBoolean()