Replace disabled linter with variable substituion

deploy/synology_dsm.sh

@@ -68,8 +68,7 @@ synology_dsm_deploy() {
 
   # Get the certificate description, but don't save it until we verfiy it's real
   _getdeployconf SYNO_Certificate
-  # shellcheck disable=SC2154
-  if [ -z "${SYNO_Certificate}" ]; then
+  if [ -z "${SYNO_Certificate:?}" ]; then
     _err "SYNO_Certificate needs to be defined (with the Certificate description name)"
     return 1
   fi
@@ -108,8 +107,7 @@ synology_dsm_deploy() {
   id=$(echo "$response" | sed -n "s/.*\"desc\":\"$SYNO_Certificate\",\"id\":\"\([^\"]*\).*/\1/p")
   _debug2 id "$id"
 
-  # shellcheck disable=SC2154
-  if [ -z "$id" ] && [ -z "$SYNO_Create" ]; then
+  if [ -z "$id" ] && [ -z "${SYNO_Create:?}" ]; then
     _err "Unable to find certificate: $SYNO_Certificate and \$SYNO_Create is not set"
     return 1
   fi