do not write a cache entry if there is no cache key

Writing to the cache with an empty key will fail with "failed to open stream: Is a directory", so do not try to do that.

Author: themasch

src/Downloader.php

@@ -293,7 +293,7 @@ private function parseJson(string $json, string $url, string $cacheKey, array $l
         }
 
         $response = new Response($data, $lastHeaders);
-        if ($response->getHeader('last-modified')) {
+        if ($response->getHeader('last-modified') && $cacheKey) {
             $this->cache->write($cacheKey, json_encode($response));
         }