Eval: Make a NullExpression evaluate to a Value with Type "NulLValue" instead of returning null.

src/main/java/de/hsrm/compiler/Klang/visitors/EvalVisitor.java

@@ -507,7 +507,9 @@ public class EvalVisitor implements Visitor<Value> {
 
   @Override
   public Value visit(NullExpression e) {
-    return null;
+    var nullValue = new Value(null);
+    nullValue.type = new NullType();
+    return nullValue;
   }
 
   @Override