ZTS: deletes home directories in /export/home

In the cleanup for the privilege tests, an empty variable, empty because
the corresponding setup is skipped on Linux, results in /export/home
being deleted. This patch adds an assertion that the variable is not
empty, and causes the cleanup to be skipped on Linux as well.

Reviewed by: John Wren Kennedy <jwk404@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Signed-off-by: John Gallagher <john.gallagher@delphix.com>
External-issue: LX-1099
Closes openzfs#7615

tests/zfs-tests/tests/functional/privilege/cleanup.ksh

@@ -31,9 +31,15 @@
 
 . $STF_SUITE/include/libtest.shlib
 
+if is_linux; then
+	log_unsupported "Privilege tests require pfexec command"
+fi
+
 verify_runnable "global"
 
 ZFS_USER=$(cat /tmp/zfs-privs-test-user.txt)
+[[ -z $ZFS_USER ]] && log_fail "no ZFS_USER found"
+
 USES_NIS=$(cat /tmp/zfs-privs-test-nis.txt)
 
 if [ "${USES_NIS}" == "true" ]