Guard against null data in AbstractLeafCandidateNode

Guard against null data in AbstractLeafCandidateNode

An instance of this class is defunct if data is ever null, as accessors
will throw a NPE from Optional.of(). Make sure data passed down is never
null, catching any possible offenders.

This transitively fixes the transformation functions, as they will throw
NPE when they get a null input, as per Function API contract.

Change-Id: I8851e3c68dc9aa5abab4874f019fd214b20c0415
Signed-off-by: Robert Varga <rovarga@cisco.com>
(cherry picked from commit 540b47bdefb25b8edb8481499efcd51d402bd57c)