BUG-650: clarify BindingNormalizedNodeSerializer API contract

Non-representable YangInstanceIdentifiers are properly reported via a
null return instead of raising an undocumented IllegalArgumentException.

Also does some implementation house-cleaning geared towards performance,
as ArrayList gives better locality.

Change-Id: Iea4717c19587fd2cc1e403aae4c5967811240e9a
Signed-off-by: Robert Varga <rovarga@cisco.com>