CASSANDRA-17857: upsert with adder support is not consistent with numbers and strings in LWT

[dcapwell]

[dcapwell]

This is probably still the best outcome, but perhaps we should raise a DISCUSS thread on the mailing list to confirm?

Fair, for my use was I prefer null + 42 = 42 rather than null (matching counter) but it is departing from SQL so should prob be more widely agreed too.