Raise error when -z option's argument is empty

When running trackbone apply -z "" -c <my_configuration>, Trackbone applies the configuration for all zones.

It would be safer to raise an error when the -z option is used with empty string as argument to prevent mistakes when running trackbone apply -z "$ZONE_NAME" -c <my_configuration> and the variable $ZONE_NAME is not set.

Of course, it would still be possible to apply a configuration on all zones by ommiting the -z option.