The code was setting GlyphClassDef.classDefs for the base font to an
empty dict then reading it from all fonts. It accidentally works when
creating variable fonts because the GlyphClassDef of the other fonts
will be used, but when mutating there is only one font.
Fix by reading the glyph classes before assigning to an empty dict.
These originated from TTFs that were built from the test UFO masters using fontmake v1.2.0 with the command `fontmake -m BuildTest.designspace -o variable`