cirrus: kill zfs dataset owner only when there is one

Commit 224f0b93d2 added the kill but it seems to not always be the
case there's process holding the datasets which ultimately leads to the
kill to fail and in turn CI to turn red.

Signed-off-by: Valentin Rothberg <vrothberg@redhat.com>

contrib/cirrus/build_and_test.sh

@@ -71,7 +71,10 @@ case $TEST_DRIVER in
         zfs create $zpool/tmp
         TMPDIR="/$zpool/tmp" showrun make STORAGE_DRIVER=$TEST_DRIVER local-test-integration local-test-unit
         # Ensure no datasets are held open prior to `zfs destroy` trap.
-        kill $(lsns -J -t mnt --output-all | jq '.namespaces[]|select(.command=="sleep 1000s").pid')
+        datasets=$(lsns -J -t mnt --output-all | jq '.namespaces[]|select(.command=="sleep 1000s").pid')
+        if [[ -n "$datasets" ]]; then
+            kill $datasets
+        fi
         ;;
     *)
         die "Unknown/Unsupported \$TEST_DRIVER=$TEST_DRIVER (see .cirrus.yml and $(basename $0))"