Add php-cs-fixer rule to prevent nullable deprecations

Author: stayallive

.php-cs-fixer.php

@@ -9,6 +9,7 @@
 $config
     ->setRules([
         '@PSR2' => true,
+        'nullable_type_declaration_for_default_null_value' => true,
     ])
     ->setFinder($finder)
 ;