fix null config

src/PaymentServiceProvider.php

         /** @noinspection PhpIncludeInspection */
         $config->set('payment', array_merge(
             require $config_path,
-            $config->get('payment')
+            $config->get('payment', [])
         ));
 
         $this->app->singleton('payment', function (Application $app) use ($config) {