[io grant] Allow no-arg &prompt
[io grant] Allow no-arg &prompt

It's not entirely unheard of to not want to include any text message
to the user when prompting for some input. It's a bit antisocial to
force the user to specify `''` arg for such cases.

Worse, they might introduce a subtle bug into their code by switching
to use `get` for a message-less `prompt`, but `get` defaults to
`$*ARGFILES`, instead of `$*IN`

softmoth/rakudo,rakudo/rakudo,awwaiid/rakudo,jonathanstowe/rakudo,tbrowder/rakudo,tbrowder/rakudo,softmoth/rakudo,salortiz/rakudo,jonathanstowe/rakudo,labster/rakudo,tony-o/rakudo,jonathanstowe/rakudo,tony-o/rakudo,MasterDuke17/rakudo,usev6/rakudo,salortiz/rakudo,rakudo/rakudo,rakudo/rakudo,Gnouc/rakudo,sjn/rakudo,Gnouc/rakudo,MasterDuke17/rakudo,raydiak/rakudo,lucasbuchala/rakudo,lucasbuchala/rakudo,jonathanstowe/rakudo,softmoth/rakudo,nbrown/rakudo,sjn/rakudo,salortiz/rakudo,nbrown/rakudo,jonathanstowe/rakudo,sjn/rakudo,Gnouc/rakudo,awwaiid/rakudo,Gnouc/rakudo,labster/rakudo,raydiak/rakudo,Gnouc/rakudo,nbrown/rakudo,awwaiid/rakudo,tbrowder/rakudo,sjn/rakudo,labster/rakudo,labster/rakudo,tbrowder/rakudo,softmoth/rakudo,sjn/rakudo,lucasbuchala/rakudo,salortiz/rakudo,tony-o/rakudo,usev6/rakudo,Gnouc/rakudo,labster/rakudo,salortiz/rakudo,raydiak/rakudo,nbrown/rakudo,tony-o/rakudo,MasterDuke17/rakudo,tbrowder/rakudo,rakudo/rakudo,rakudo/rakudo,nbrown/rakudo,awwaiid/rakudo,raydiak/rakudo,labster/rakudo,MasterDuke17/rakudo,tbrowder/rakudo,tony-o/rakudo,awwaiid/rakudo,usev6/rakudo,usev6/rakudo,usev6/rakudo,lucasbuchala/rakudo,nbrown/rakudo,lucasbuchala/rakudo,salortiz/rakudo,MasterDuke17/rakudo,softmoth/rakudo,rakudo/rakudo,MasterDuke17/rakudo,tony-o/rakudo