Previously otlLib was generating None if the values themselves were
empty even if the value format was non-empty. This happened to work
for compiling to binary since the compiler handles Value=None.
But this was confusing varLib.merger module (as in when building
variable fonts from such otlLib-built master GSUB/GPOS tables, without
roundtripping to OTF/TTF binary first), because in varLib.merger,
a None means "this master doesn't provide that info; skip it"; whereas
in a PairPos table a None as generated by otlLib simply meant "all
values are zero", which is different from "this master doesn't
provide this value".
This fixes that, such that ufo2ft can build variable-font without
saving masters to binary.
Part of https://github.com/googlefonts/ufo2ft/pull/486
0faf01d5f1