mgmtd: fix printing an empty data tree

mgmtd: fix printing an empty data tree

We don't need to create an actual tree to print an empty tree, libyang
handles NULL just fine. The actual problem is that `yang_dnode_new`
creates a tree by validating it, and the validation creates all implicit
default nodes. Therefore, when called with "with-default" flags, instead
of getting an empty tree, we get a tree with all top-level default set.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>