Use "hasKey" instead of checking the value

If the check is negative it would depending on the used cache store the value as an empty string. When reading the value this check would thus return "false" even if a value exists.

apps/files_sharing/lib/external/storage.php

@@ -253,8 +253,8 @@ class Storage extends DAV implements ISharedStorage {
 	 */
 	private function testRemoteUrl($url) {
 		$cache = $this->memcacheFactory->create('files_sharing_remote_url');
-		if($result = $cache->get($url)) {
-			return (bool)$result;
+		if($cache->hasKey($url)) {
+			return (bool)$cache->get($url);
 		}
 
 		$result = file_get_contents($url);