Mark parameter type as nullable.

This only pops up in github ci when you fully upgrade php-cs-fixer, as this is a new check.

Author: doppynl

src/NamespaceVisibility.php

@@ -8,7 +8,7 @@
 final class NamespaceVisibility
 {
     public function __construct(
-        string $namespace = null,
+        ?string $namespace = null,
         bool $excludeSubNamespaces = false,
     ) {
     }