Fix delete operation for leaf nodes

Fix delete operation for leaf nodes

In case of delete operation we do not have
NormalizedNode representing data.

To create NormalizedNode we cannot use
ImmutableNodes.fromInstanceId() because it violates
leaf node non-null value contract.

Instead we have to emit data to delete using
EmptyListXmlWriter the similar way as its
done with filters.

Similar way we have to emit data together
with metadata using EmptyListXmlMetadatWriter
to overcome NormalizedMetadataWriter' write
method which requires data in the form of NormalizedNode.

JIRA: NETCONF-833
Change-Id: I2dc2921a78e7fc41d1d5eda101978c3a2e36ec12
Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>