cli: fix crash on autocompletion

@connections is NULL when doing autocompletion. Fixes the following:

 $ nmcli --complete-args con monitor ""
  help
  id
  uuid
  path
  filename
  ...
  Segmentation fault (core dumped)

Fixes: 4b329727 ('cli: rework connection handling for multiple results')

https://bugzilla.redhat.com/show_bug.cgi?id=1716948
14 jobs for bg/cli-monitor-crash-rh1716948 in 44 minutes and 28 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #339305
allowed to fail
checkpatch

00:01:00

manual #339310
allowed to fail manual
t_centos:7.5.1804
passed #339311
t_centos:7.6.1810

00:32:03

passed #339314
t_debian:9

00:25:40

passed #339316
t_debian:sid

00:13:19

manual #339315
allowed to fail manual
t_debian:testing
passed #339306
t_fedora:28

00:39:21

passed #339307
t_fedora:29

00:44:25

passed #339308
t_fedora:30

00:39:21

manual #339309
allowed to fail manual
t_fedora:rawhide
passed #339312
t_ubuntu:16.04

00:25:07

passed #339313
t_ubuntu:18.04

00:25:48

 
  External
passed NetworkManager test verification

passed NetworkManager test verification

05:24:56