null for a blank value, matching encodeCell.
The counterpart to encodeCell for the stateless value layer (IRange, Scalar[][]),
where there is no cell to ask for text and therefore no number format to apply. That is
the whole difference: 0.04 encodes as 0.04 here and as 4% through a cell that formats
it. There is consequently no mode — this is what CellEncoding.Mode.Value produces
for a cell, and the only form a bare value has.
Everything above formatting is identical, because it is the same contract: the ' prefix on
strings, TRUE/FALSE, error labels, blank elision.
Encodes one bare Scalar — a value with no cell behind it.