check for unset variable correctly

start.sh

@@ -23,7 +23,7 @@ setup_ldap_source() {
 
 # SSH_PORT can be unset to disable SSH
 disable_ssh="false"
-if [[ -z "${SSH_PORT}" ]]; then
+if [[ -z "${SSH_PORT:-}" ]]; then
     echo "SSH disabled"
     SSH_PORT=29418 # arbitrary port to keep sshd happy
     disable_ssh="false"