Add support for `empty` YANG leaf type

Change-Id: I87eafae9df9accdaa4579ace769996e70da6cb1c
diff --git a/tests/example-schema.yang b/tests/example-schema.yang
index 5d84eda..ef9d02d 100644
--- a/tests/example-schema.yang
+++ b/tests/example-schema.yang
@@ -243,4 +243,8 @@
     leaf blob {
         type binary;
     }
+
+    leaf dummy {
+        type empty;
+    }
 }