Simplify cgroup_mount error checking logic

Remove redundant conditional as suggested in #599 (review)

Author: gberche-orange

stemcell_builder/stages/bosh_monit/assets/monit-access-helper.sh

@@ -32,7 +32,7 @@ permit_monit_access() {
         cgroup_mount="$(awk '$1 == "cgroup2" && $3 == "cgroup2" { print $2 }' /proc/self/mounts)"
         nb_matching_cgroup_mounts=$(echo "$cgroup_mount" | grep -c '^.')
         current_cgroup="$(grep '^0::' /proc/self/cgroup | cut -d: -f3)"
-        if [ -z "${cgroup_mount}" ] || [ "${nb_matching_cgroup_mounts}" -ne 1 ] || [ -z "${current_cgroup}" ]; then
+        if [ "${nb_matching_cgroup_mounts}" -ne 1 ] || [ -z "${current_cgroup}" ]; then
             echo "permit_monit_access: unable to resolve cgroup v2 mount or path. current_cgroup=${current_cgroup} cgroup_mount=${cgroup_mount} nb_matching_cgroup_mounts=${nb_matching_cgroup_mounts}" >&2
             return 1
         fi