fix(build): skip empty gateway value in network config

When var_gateway is set to an empty string, the resulting gw= token
in the comma-separated network string causes pct create to fail with
a 'missing key in comma-separated list property' error.

Closes #13421

misc/build.func

@@ -3530,6 +3530,7 @@ build_container() {
   # Gateway
   if [[ -n "$GATE" ]]; then
     case "$GATE" in
+    ,gw=) ;;
     ,gw=*) NET_STRING+="$GATE" ;;
     *) NET_STRING+=",gw=$GATE" ;;
     esac