[ttfcomponentizer] Add warning when UFO public.postscriptNames key is present but empty

[josh-hadley]

The presence/absence of the public.postscriptNames key in the UFO lib.plist used during the componentization process can have a major impact on how components are made, as it determines which glyph names will be used to select components. An empty dict for this key is assumed to indicate no difference between UFO and TTF glyph names, but currently no indication is given that this assumption is being acted upon. A warning for the case of an empty public.postscriptNames lib key could be useful for cases where an empty dict is unintentionally included in lib.plist. Addition of this warning should not change the overall behavior of ttfcomponentizer beyond printing a warning in the output.

Suggested wording for the warning:

WARNING: The contents of public.postscriptNames is empty. This may result in uncomponentized glyphs if final glyph names vary from production names.