Fixed getIdentifier() must be of the type string (#7)

When `$configuration->getName()` is `null`, which is possible with the default `@ParamConverter` from `FrameworkExtraBundle` an exception will be thrown because `getIdentifier` requires a string as third param. 
Using `?string $name` is not allowed since the bundle should be compatible with php >= 7.1

`private function getIdentifier(Request $request, $options, string $name): string`

src/ParamConverter/HashidsParamConverter.php

@@ -45,7 +45,7 @@ private function setHashid(Request $request, ParamConverter $configuration): voi
             $this->getIdentifier(
                 $request,
                 array_replace(['hashid' => null], $configuration->getOptions()),
-                $configuration->getName()
+                (string) $configuration->getName()
             )
         );