Fix GSUB/GPOS/GDEF compile
Fix GSUB/GPOS/GDEF compile

I'm not sure how it was every working.  If mti_paths is {}, then

  mtiFeaFiles=(mti_paths and mti_paths.get(name)

will set mtiFeaFiles to {}, whereas ufo2ft expects None.

Ie, we were getting no GSUB/GPOS/GDEF for any fonts that did NOT
have mti sources.

googlei18n/fontmake,googlefonts/fontmake,googlefonts/fontmake,googlei18n/fontmake