MINOR: cli: automatically enable a CLI I/O handler when there's no parser

MINOR: cli: automatically enable a CLI I/O handler when there's no parser

Sometimes a registered keyword will not need any specific parsing nor
initialization, so it's annoying to have to write an empty parsing
function returning zero just for this.

This patch makes it possible to automatically call a keyword's I/O
handler of when the parsing function is not defined, while still allowing
a parser to set the I/O handler itself.