(follow-up on #1399) Support ignoring of null when merging

[cowtowncoder]

(off-shoot of #1399)

One edge case with merging is that incoming value of null gets handled before deserializer, and can thereby overwrite existing value. This is probably not what user wants; and although we'll tackle some aspects with #1402, the most straight-forward solution would seem to be to just ignore incoming null when merging.
So let's see if that is doable.