docs: remove file header with license reference

Author: DannyvdSluijs

tests/Constraints/UnionWithNullValueTest.php

@@ -1,12 +1,5 @@
 <?php
 
-/*
- * This file is part of the JsonSchema package.
- *
- * For the full copyright and license information, please view the LICENSE
- * file that was distributed with this source code.
- */
-
 namespace JsonSchema\Tests\Constraints;
 
 class UnionWithNullValueTest extends BaseTestCase