Correct typehint style

Typehints of parameters, that have a default value of `null` do not
have to be marked nullable explicitly, i.e. `?string` can be just
`string`.

src/CodeGenerator/InjectorGenerator.php

@@ -79,7 +79,7 @@ class InjectorGenerator
     public function __construct(
         ConfigInterface $config,
         DependencyResolverInterface $resolver,
-        ?string $namespace = null,
+        string $namespace = null,
         LoggerInterface $logger = null
     ) {
         $this->config = $config;