Reset the program script during installation contribution

If not the script code got set during the first invocation (after bootup?)
So, there was no chance to fetch the code once a connection could not be
established, as the popup code was already retreived and the check for
an empty string failed.

src/main/java/com/fzi/externalcontrol/impl/ExternalControlInstallationNodeContribution.java

@@ -65,7 +65,8 @@ public boolean isDefined() {
 
   @Override
   public void generateScript(ScriptWriter writer) {
-
+    // Make sure the program gets requested at least once
+    urScriptProgram = "";
   }
 
   // IP helper functions