BUG/MINOR: cli: don't complain about empty command on empty lines

BUG/MINOR: cli: don't complain about empty command on empty lines

There's something very irritating on the CLI, when just pressing ENTER,
it complains "Unknown command: ''..." and dumps all the help. This
action is often done to add a bit of clearance after a dump to visually
find delimitors later, but this stupid error makes it unusable. This
patch addresses this by just returning on empty command instead of trying
to look up a matching keyword. It will result in an empty line to mark
the end of the empty command and a prompt again.

It's probably not worth backporting this given that nobody seems to have
complained about it yet.