[math] Don’t drop table with no glyph construction
[math] Don’t drop table with no glyph construction

For example Caudex [1] has MATH table with 0 VertGlyphCount and
HorizGlyphCount in MathVariants table, but that is no reason to reject
it.

1. https://github.com/google/fonts/tree/master/ofl/caudex

anthrotype/ots,anthrotype/ots,irori/ots,khaledhosny/ots,davelab6/ots,davelab6/ots,irori/ots