lib: fix oper data leaf creation

When creating an initial tree trunk for oper data walk, if the xpath
represents a leaf, the leaf is created with an incorrect empty value.
If it doesn't actually exist in daemon's oper data, its value is not
overwritten later and an empty value is returned in the result.

For example, when requesting
`/frr-interface:lib/interface[name='eth0']/description`, the result is:
```
{
  "frr-interface:lib": {
    "interface": [
      {
        "name": "eth0",
        "description": ""
      }
    ]
  }
}
```
instead of an empty JSON that it should be.

Signed-off-by: Igor Ryzhov <[email protected]>

lib/northbound_oper.c

@@ -515,8 +515,18 @@ static enum nb_error nb_op_ys_init_node_infos(struct nb_op_yield_state *ys)
 
 	/* Move up to the container if on a leaf currently. */
 	if (node &&
-	    !CHECK_FLAG(node->schema->nodetype, LYS_CONTAINER | LYS_LIST))
+	    !CHECK_FLAG(node->schema->nodetype, LYS_CONTAINER | LYS_LIST)) {
+		struct lyd_node *leaf = node;
+
 		node = &node->parent->node;
+
+		/*
+		 * If the leaf is not a key, delete it, because it has a wrong
+		 *  empty value.
+		 */
+		if (!lysc_is_key(leaf->schema))
+			lyd_free_tree(leaf);
+	}
 	assert(!node ||
 	       CHECK_FLAG(node->schema->nodetype, LYS_CONTAINER | LYS_LIST));
 	if (!node)