BUG 8927: Netconf response payload fails to render in JSON

BUG 8927: Netconf response payload fails to render in JSON

JSONNormalizedNodeStreamWriter does not write a content of anyXml nodes
correctly mainly due to Text nodes containing whitespaces before, between
or after Element nodes. Therefore the first Text node only was serialized
to JSON and the rest of anyXML content was missing.
This patch provides fix of anyXml content serialization to JSON.

Added a fix for a null pointer for empty text node.

Change-Id: I74f34aecec1a85b2ede4be63e5b6dd74522981e4
Signed-off-by: Peter Kajsa <pkajsa@cisco.com>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
Signed-off-by: Atul Gosain <agosain@luminanetworks.com>