Extra handling of null values (part 1) (#684)

src/main/java/carpet/script/value/NullValue.java

@@ -40,6 +40,21 @@ public class NullValue extends NumericValue // TODO check nonsingleton code
         return o instanceof NullValue;
     }
 
+    @Override
+    public Value slice(long fromDesc, Long toDesc) {
+        return Value.NULL;
+    }
+
+    @Override
+    public NumericValue opposite() {
+        return Value.NULL;
+    }
+
+    @Override
+    public int length() {
+        return 0;
+    }
+
     @Override
     public int compareTo(Value o)
     {