Add nullable_type_declaration rule

Author: natanfelles

src/Config.php

@@ -201,6 +201,7 @@ protected function prepareRules() : void
             'normalize_index_brace' => true,
             'not_operator_with_space' => false,
             'not_operator_with_successor_space' => false,
+            'nullable_type_declaration' => true,
             'nullable_type_declaration_for_default_null_value' => true,
             'object_operator_without_whitespace' => true,
             'ordered_class_elements' => [