acceptNull might be redundant following #243

[edwardsmarc]

Now we have #243, there might not be any occasions where a null inpur value needs to be passed through a validaiton function.

We also have some new functions that might replace the need for acceptNull.

e.g. two matchList calls with acceptNull = TRUE can be replaced with MatchListTwice
or
matchList(coalesce(val1, val2), ...)