fontinfo.plist import/export v4. update for PR 215

see https://github.com/metapolator/metapolator/pull/215
I remember when ufoJS tried to return an empty glyphset when there was no folder for that, this was an error in some cases. Actually, it got filed and fixed: graphicore/ufoJS#7 and also #134
This is remarkably similar. Based on that, I don't think, returning just an empty object is the right thing to do.