Set null type to be abstract (#460).

plugins/org.yakindu.base.types/src/org/yakindu/base/types/typesystem/GenericTypeSystem.java

@@ -31,5 +31,6 @@ public class GenericTypeSystem extends AbstractTypeSystem {
 
 		declareSuperType(getType(INTEGER), getType(REAL));
 		
+		getType(NULL).setAbstract(true);
 	}
 }