added default ctor for tuples

src/main/jastadd/base/data/Tuples.jadd

@@ -8,6 +8,9 @@ public class Tuple2<K, V> {
         this._2 = _2;
     }
 
+    public Tuple2(){
+    }
+
     public K get_1() {
         return _1;
     }