Clean-up: kern table: only write pairs where both elements have Unicode value

[madig]

FontVal complains about many, many kern pairs where one element does not show up in the cmap table (e.g. Parenleft, Guilsinglright). These have no Unicode value assigned and are therefore never seen by OpenType-feature-unaware applications. Filter them out.

[moyogo]

Ouch! Indeed, some apps using the kern table don't use it when this happens.