fix unbound variable that throw an ERROR during CI action

Author: gmella

build_gui.sh

@@ -12,6 +12,7 @@ cd -
 
 # note: use insecure https as restlet https certificates are out-dated!
 #MVN_OPTS="-Dmaven.wagon.http.ssl.insecure=true -Dmaven.wagon.http.ssl.allowall=true -Dmaven.wagon.http.ssl.ignore.validity.dates=true"
+MVN_OPTS=""
 echo "MVN_OPTS: $MVN_OPTS"
 
 # Build all modules (skip test)