Add support for `empty` YANG leaf type

Change-Id: I87eafae9df9accdaa4579ace769996e70da6cb1c
diff --git a/src/utils.cpp b/src/utils.cpp
index 665aedd..3543787 100644
--- a/src/utils.cpp
+++ b/src/utils.cpp
@@ -123,6 +123,10 @@
     {
         return "a leafref";
     }
+    std::string operator()(const yang::Empty&)
+    {
+        return "an empty leaf";
+    }
     std::string operator()(const yang::Union& type)
     {
         std::ostringstream ss;
@@ -163,6 +167,11 @@
         return data.m_value;
     }
 
+    std::string operator()(const empty_) const
+    {
+        return "[empty]";
+    }
+
     std::string operator()(const identityRef_& data) const
     {
         return data.m_prefix ? (data.m_prefix.value().m_name + ":" + data.m_value) : data.m_value;