Allow omitting channels argument from module-jackdbus-detect
If you don't specify a number of channels argument to module-jack-sink or module-jack-source, they use the number of physical output/input ports (respectively) registered in the JACK server. module-jackdbus-detect ostensibly allows you to omit its channels argument and it will load module-jack-sink and module-jack-source without passing them any channels argument. However, this doesn't work because module-jackdbus-detect uses a default channels value of 0 to indicate this, and the channels argument (even if it is the default because it was unspecified) is passed to pa_channels_valid which fails on 0. This check needs to be fixed so omitting the channels argument from module-jackdbus-detect is considered valid.