Avoid bash error when CUPROTOCOMPAT is unset

[sparr]

% steamtinkerlaunch vortex install
/bin/steamtinkerlaunch: line 4406: [: : integer expression expected

This error occurs when $CUPROTOCOMPAT is unset. The error does not seem to prevent the script from working.

This PR checks whether it's empty before checking its value.

I'm a bit fuzzy on how to update PROGVERS here and would appreciate some guidance if this is to be accepted.

[sonic2kk]

This is great, thank you! Sorry for the delay with getting back, holidays and such 😄

About PROGVERS, just bump it to the current day. For example, v14.0.20241228-1 would be perfectly fine :-)

[sonic2kk]

Many thanks for your contribution, it is much appreciated! Changelog has also been updated to give credit: https://github.com/sonic2kk/steamtinkerlaunch/wiki/Changelog/_compare/a262ad1ba229bd9b161a38ad136a27059185a4f1