BUG/MEDIUM: cli: Don't look for payload pattern on empty commands

BUG/MEDIUM: cli: Don't look for payload pattern on empty commands

A regression was introduced by commit 9431aa0bdf ("BUG/MEDIUM: cli: Don't
look for payload pattern on empty commands").

On empty commands (really empty or containing spaces and tabs), the number of
arguments set to 0. However we look for the payload pattern without checking
it. The result is an access at the index -1 in the argument array. It is of
course invalid.

To fix the issue, we just skip this part when there is no argument. Note that
the empty command is still sent to the worker.

This patch should solve the issue #2365. No backport needed.