VariationMerger is updated such that it accepts None list members
in .mergeThings, and switches to correct sub-model. This means,
it technically supports more than just a few masters missing GPOS.
This allows for some within-GPOS differences to also be handled,
though that's not well-studied or complete.
Part of https://github.com/googlei18n/fontmake/issues/88
Part of https://github.com/fonttools/fonttools/issues/1355
the minimalist STAT table that varLib produces only contains one DesignAxisRecord
for each fvar axis, and has AxisValueCount=0, thus the AxisValueArray offset is None.