glyphs2ufo, Fontinfo: do not write out empty values unless they mean something

E.g. postscriptBlueValues and postscriptOtherBlues just clutter the fontinfo when they're empty.

Other empty lists like openTypeOS2Type have a meaning.

Locate the glyphs2ufo and Fontinfo serialization code that handles postscriptBlueValues, postscriptOtherBlues, and openTypeOS2Type. Inspect existing tests for empty-list output and add coverage for the distinction described in the issue. Done means empty values without meaning are omitted while meaningful empty lists remain written.

Written by the indexing model from the issue text.