Fix empty public.postscriptNames dict incorrectly triggering uniXXXX derivation

[anthrotype]

When a UFO's lib.plist contains public.postscriptNames as an empty dict (<dict/>), ufo2ft incorrectly derives uniXXXX/uXXXXX production names for all glyphs. But an empty mapping should mean "no glyphs are renamed," same as if it were absent.

The Georama family has <key>public.postscriptNames</key><dict/> in all 7 masters. fontmake renames every glyph to uniXXXX; fontc correctly keeps the source names.

The problem is the is not None check in process_glyph_names(): an empty dict {} passes that check, so useProductionNames is set to True. Then in _build_production_name(), the truthiness guard if self._postscriptNames: skips the (empty) mapping and incorrectly falls through to unicode-based derivation.

The fix changes is not None to a truthiness check so empty and absent are treated the same, matching the documented semantics from commit 2f800cb:

If the mapping is not present, no glyph names are renamed.

There is already a way to explicitly opt into uniXXXX renaming by setting the com.github.googlei18n.ufo2ft.useProductionNames lib key to true (or by passing --production-names to fontmake CLI in absence of an explicit public.postscriptNames dict in the lib):

<key>com.github.googlei18n.ufo2ft.useProductionNames</key>
<true/>

An empty public.postscriptNames dict was never supposed to implicitly trigger the same behavior.

[anthrotype]

Note: the v2.4.0 release notes (https://github.com/googlefonts/ufo2ft/releases/tag/v2.4.0) already documented this as the intended behavior:

if a public.postscriptNames mapping is not provided in the UFO lib, then do not do any glyph renaming.

The empty-dict case was an oversight in the implementation.