VF edge case: GPOS is dropped when default source has no kerning but other masters do

[madig]

Reproducer: Kern.zip -- GPOS exists in Light and Bold interpolatable masters, but is dropped in resulting VF because the default source is Regular and has no kerning.

An edge case for sure, but touches the question of what should happen when some masters have a kern pair but not others: fill in a zero or interpolate value between the other masters (and extrapolate for "outside" masters?!)?