Do not use null input for discard-changes

Do not use null input for discard-changes

There is already a NormalizedNode defined in NetconfMessageTransformUtil
for the content of the 'discard-changes' RPC.

Use the static variable instead of 'null'. This would make message
transformation more robust and fix an error caused by an augmentation
to the RPC input while generating NetconfMessage.

JIRA: NETCONF-934
Change-Id: I38777313d74e92bd955248ceea78e765729d9259
Signed-off-by: Sangwook Ha <sangwook.ha@verizon.com>
(cherry picked from commit f3cd4750c64f9002940b3bf20ea6be47a14344c3)
(cherry picked from commit 7b62bfe36a7ce71c0952799b169b37df6c269f5e)