LU-10338 test: handle empty CLIENTS in mountcli()

LU-10338 test: handle empty CLIENTS in mountcli()

In mountcli() use if [ -n "$CLIENTS" ]; ... rather than
[ -n "$CLIENTS" ] && ... to avoid immediate exit on error when CLIENTS is
the empty string.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I440c3eefb4bee134caee1db5ff2ecdbdf9c1ee5c
Reviewed-on: https://review.whamcloud.com/30413
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>