implement visitors for null atom

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

@@ -496,4 +496,10 @@ public class EvalVisitor implements Visitor<Value> {
     return null;
   }
 
+  @Override
+  public Value visit(NullExpression e) {
+    // TODO Auto-generated method stub
+    return null;
+  }
+
 }

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

@@ -819,4 +819,10 @@ public class GenASM implements Visitor<Void> {
     return null;
   }
 
+  @Override
+  public Void visit(NullExpression e) {
+    // TODO Auto-generated method stub
+    return null;
+  }
+
 }

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

@@ -260,4 +260,9 @@ class GetVars implements Visitor<Void> {
     return null;
   }
 
+  @Override
+  public Void visit(NullExpression e) {
+    return null;
+  }
+
 }

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

@@ -432,4 +432,10 @@ public class PrettyPrintVisitor implements Visitor<Void> {
     return null;
   }
 
+  @Override
+  public Void visit(NullExpression e) {
+    ex.write("null");
+    return null;
+  }
+
 }

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

@@ -47,4 +47,5 @@ public interface Visitor<R> {
     R visit(StructField e);
     R visit(StructFieldAccessExpression e);
     R visit(ConstructorCall e);
+    R visit(NullExpression e);
 }