Only apply Aws Credentials if they exist. Otherwise, allow driver to …

…use Instance Iam Profile.

src/Illuminate/Mail/TransportManager.php

@@ -109,7 +109,7 @@ protected function createSesDriver()
      */
     protected function addSesCredentials(array $config)
     {
-        if ($config['key'] && $config['secret']) {
+        if (!empty($config['key']) && !empty($config['secret'])) {
             $config['credentials'] = Arr::only($config, ['key', 'secret', 'token']);
         }