scalebar transform default should not be NULL (throws error)

[brianstock]

Hi,

My plotting code that used ggsn::scalebar fine last year stopped working until I made a minor change, replacing dd2km with transform. The documentation doesn't explain this.

See https://stackoverflow.com/questions/58510360/i-am-looking-to-add-a-scalebar-to-my-ggmap-in-r-but-keep-getting-transform-sho/62901185#62901185.

Thanks for the package - otherwise works great!