Fix type error in ResultPager::fetch

When using etags, the reply from github may be empty and the underlying APIs will return an empty string.
We need to flip those to empty arrays.
e.g.:
```
$github_builder
  ->addPlugin(new Http\Client\Common\Plugin\HeaderSetPlugin([
    'If-None-Match' => $etag,
  ]));

$api       = $github_client->user('user');
$paginator = new \Github\ResultPager($github_client);
$data      = $paginator->fetch($api, 'events', [$username]);
// $data should be [] if $etag is the latest
```

lib/Github/ResultPager.php

@@ -88,7 +88,7 @@ public function fetch(AbstractApi $api, string $method, array $parameters = []):
 
         $this->postFetch(true);
 
-        return $result;
+        return $result ? $result : [];
     }
 
     /**