BUG-1704: do not emit separator

When reusing JSONNormalizedNodeStreamWriter, it retains enough state to
know to emit a separator, so if we do it here, we will break the output.

Change-Id: Ie7acc269b4546230fad532f0abf14c0ba6c4121a
Signed-off-by: Robert Varga <rovarga@cisco.com>