Empty Setting in Audio and Video causing an error

[GAS85]

Steps to reproduce

1. As mentioned in Some Settings (Proxy User, Password and Audio Format) being ignored or not sent to the youtube-dl #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

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)

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:

/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