When no subcommands are registered, omit command help output

For a single root command with a Run method, the help output still
contains 'help [command]' as a subcommand (because Help is always
added). Since the only subcommand would be 'help', the help is better
off omitted.

This change allows a command to be used both as a subcommand
or a root command without having to define a custom help that elides
the help command when no subcommands are added.  Instead, the default
help command is only added when subcommands are present.