Empty Setting in Audio and Video causing an error

[GAS85]

### Steps to reproduce 1. As mentioned in #187 go to Admin Settings > Additional > ocDownloader 2. Live empty Audio and Video fields:  3. See an error "Unable to get correct audio URL YouTube"  ### Expected behaviour Video still downloaded with default settings - no setting for audio and video: Command to execute should looks like ```bash sh -c /usr/local/bin/youtube-dl -i 'https://www.youtube.com/watch?v=xxxxxxxxxx' --get-url --get-filename -4 ``` ### Actual behaviour Faulty command will be produced that breaks youtube-DL execution. `-f` should not be pushed without any parameter in this case (see Expected behaviour) ```bash sh -c /usr/local/bin/youtube-dl -i 'https://www.youtube.com/watch?v=xxxxxxxxxx' --get-url --get-filename -f -4 ``` Command line error will be produced: ```bash /usr/local/bin/youtube-dl -i 'https://www.youtube.com/watch?v=xxxxxxxxxx' --get-url --get-filename -f -4 ERROR: requested format not available ``` ### Server configuration **Operating system**: Ubuntu 20.04 **Nextcloud version:** 21.04 #### ocDownloader configuration: **Which downloader are you using:** Curl **Do you use a proxy:** No **Which protocols do you allow**: FTP/HTTP/Youtube