Fixed reading whole list/leaf-list using GET/GET-CONFIG RPC

Fixed reading whole list/leaf-list using GET/GET-CONFIG RPC

- The source of the issue was in skipping of empty list/leaf-list
  nodes in XMLStreamNormalizedNodeStreamWriter - the NETCONF
  filter structure was created correctly, but it wasn't correctly
  serialized into XML. However, in NETCONF, it is valid to read
  whole list/leaf-list - we must be able to build NormalizedNode
  structure from subtree-filter which "ends" by list/leaf-list.
- Fixed by introduction EmptyListXmlWriter which is responsible
  for serialization of empty list/leaf-list. Other operations
  are delegated to XMLStreamNormalizedNodeStreamWriter
  implementation.

JIRA: NETCONF-744
Change-Id: Id4b665aa45a397ab25a53e12f4eb7c1a6539d428
Signed-off-by: Jaroslav Tóth <jtoth@frinx.io>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>