Fix NetconfAccess::listInstances segfault

The segfault happened when no instances were returned by getConfig.

Change-Id: Ia3765cf04e635d58d665120a3a77d81446901108
diff --git a/tests/data_query.cpp b/tests/data_query.cpp
index 1fd461c..e0e2875 100644
--- a/tests/data_query.cpp
+++ b/tests/data_query.cpp
@@ -75,6 +75,14 @@
             };
         }
 
+        SECTION("example-schema:person - no instances")
+        {
+            node.first = "example-schema";
+            node.second = "person";
+            expected = {
+            };
+        }
+
         SECTION("example-schema:selectedNumbers")
         {
             datastore.createListInstance("/example-schema:selectedNumbers[value='45']");