Use right rule name

templates/cs/.php-cs-fixer-8.1.dist.php

@@ -17,7 +17,7 @@
         'ternary_to_null_coalescing' => true,
         'assign_null_coalescing_to_coalesce_equal' => true,
         'single_line_throw' => false,
-        'nullable_type_declaration' => true,
+        'nullable_type_declaration_for_default_null_value' => true,
     ])
     ->setFinder($finder)
 ;