fix #2578: check if $target is provided in clone

fix #2578: check if $target is provided in clone

regression introduced with commit a85ff91b

previously we set $target to undef if it's localnode or localhost, then
we check if node exists.

with regression commit, behaviour changes as we do the node check in
else, but $target may be undef. this causes an error:

    no such cluster node ''

Signed-off-by: Oguz Bektas <o.bektas@proxmox.com>
improved readability

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>