Fix check for cache value in JSCombiner
Fix check for cache value in JSCombiner

* fixes following log output, because there was empty string
  stored in the cache

Invalid argument supplied for foreach() at lib/private/Template/JSCombiner.php#108

Signed-off-by: Morris Jobke <7f550a9f4c44173a37664d938f1355f0f92a47a7@morrisjobke.de>

andreas-p/nextcloud-server,andreas-p/nextcloud-server,pixelipo/server,pixelipo/server,michaelletzgus/nextcloud-server,andreas-p/nextcloud-server,endsguy/server,pixelipo/server,endsguy/server,andreas-p/nextcloud-server,pixelipo/server,nextcloud/server,nextcloud/server,michaelletzgus/nextcloud-server,nextcloud/server,endsguy/server,michaelletzgus/nextcloud-server,michaelletzgus/nextcloud-server,pixelipo/server,andreas-p/nextcloud-server,nextcloud/server,endsguy/server,endsguy/server