Fix phpstan follow error.

src/Http/Client/Auth/Oauth.php

@@ -48,6 +48,8 @@ class Oauth
             $credentials['method'] = 'hmac-sha1';
         }
         $credentials['method'] = strtoupper($credentials['method']);
+
+        $value = null;
         switch ($credentials['method']) {
             case 'HMAC-SHA1':
                 $hasKeys = isset(