cli: cast NULL to (const char*) in variadic functions

Using NULL as the last argument for a varadic function is undefined and
leads to passing a (void*) which is undefined
20 jobs for frazer/variadicFunction in 39 minutes and 34 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
passed #3580716
checkpatch

00:01:11

manual #3580723
allowed to fail manual
t_centos:7.5.1804
manual #3580724
allowed to fail manual
t_centos:7.6.1810
manual #3580725
allowed to fail manual
t_centos:7.7.1908
manual #3580726
allowed to fail manual
t_centos:7.8.2003
manual #3580727
allowed to fail manual
t_centos:8.1.1911
manual #3580733
allowed to fail manual
t_debian:10
manual #3580732
allowed to fail manual
t_debian:9
manual #3580735
allowed to fail manual
t_debian:sid
manual #3580734
allowed to fail manual
t_debian:testing
manual #3580717
allowed to fail manual
t_fedora:28
manual #3580718
allowed to fail manual
t_fedora:29
manual #3580719
allowed to fail manual
t_fedora:30
passed #3580720
t_fedora:31

00:39:33

manual #3580721
allowed to fail manual
t_fedora:32
manual #3580722
allowed to fail manual
t_fedora:rawhide
manual #3580728
allowed to fail manual
t_ubuntu:16.04
manual #3580729
allowed to fail manual
t_ubuntu:18.04
manual #3580731
allowed to fail manual
t_ubuntu:devel
manual #3580730
allowed to fail manual
t_ubuntu:rolling