Add support for `empty` YANG leaf type

Change-Id: I87eafae9df9accdaa4579ace769996e70da6cb1c
diff --git a/src/leaf_data.hpp b/src/leaf_data.hpp
index 2f84a05..4330d5a 100644
--- a/src/leaf_data.hpp
+++ b/src/leaf_data.hpp
@@ -117,6 +117,10 @@
     {
         return leaf_data_string.parse(first, last, ctx, rctx, attr);
     }
+    bool operator()(const yang::Empty) const
+    {
+        return x3::attr(empty_{}).parse(first, last, ctx, rctx, attr);
+    }
     template <typename Type>
     void createSetSuggestions(const Type& type) const
     {