(#5552) Display help when no subcommand is given.

Previously, when the command line was empty we would try and invoke an empty
method; this was less helpful than telling people what they could actually do,
so we adapt our code to do precisely that.

Paired-With: Jesse Wolfe <jesse@puppetlabs.com>
Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>