Explicitly mark nullable parameter (#1034)

Author: cedric-anne

src/Provider/AbstractProvider.php

@@ -495,7 +495,7 @@ public function getAuthorizationUrl(array $options = [])
      */
     public function authorize(
         array $options = [],
-        callable $redirectHandler = null
+        ?callable $redirectHandler = null
     ) {
         $url = $this->getAuthorizationUrl($options);
         if ($redirectHandler) {