Avoid error when CUPROTOCOMPAT is empty (#1204)

Author: sparr

steamtinkerlaunch

@@ -7,7 +7,7 @@
 PREFIX="/usr"
 PROGNAME="SteamTinkerLaunch"
 NICEPROGNAME="Steam Tinker Launch"
-PROGVERS="v14.0.20241025-1"
+PROGVERS="v14.0.20241228-1"
 PROGCMD="${0##*/}"
 PROGINTERNALPROTNAME="Proton-stl"
 SHOSTL="stl"
@@ -4403,7 +4403,7 @@ function needNewProton {
 }
 
 function dlCustomProton {
-	if [ "$CUPROTOCOMPAT" -eq 1 ]; then
+	if [[ -n "$CUPROTOCOMPAT" && "$CUPROTOCOMPAT" -eq 1 ]]; then
 		CUPROEXTDIR="$STEAMCOMPATOOLS"
 	else
 		CUPROEXTDIR="$CUSTPROTEXTDIR"