Make more nullable types explicit

Extractor/ReflectionExtractor.php

@@ -681,7 +681,7 @@ private function getMutatorMethod(string $class, string $property): ?array
                         continue;
                     }
 
-                    // Parameter can be optional to allow things like: method(array $foo = null)
+                    // Parameter can be optional to allow things like: method(?array $foo = null)
                     if ($reflectionMethod->getNumberOfParameters() >= 1) {
                         return [$reflectionMethod, $prefix];
                     }