ufo2ft will always create cmap 4 subtables, even if empty

Whereas we will skip these subtables if they would contain no glyphs.

This shows up with some noto sources, e.g.

```
python3 -m ttx_diff 'https://github.com/notofonts/ottoman-siyaq-numbers?b6cca8ffe6#sources/NotoSerifOttomanSiyaq.glyphs'
```

Start by running the provided ttx_diff command against the Noto Serif Ottoman Siyaq source and inspect the cmap subtables it reports. Trace the relevant font compilation behavior in fontc, then verify that cmap format 4 subtables are omitted when they contain no glyphs while non-empty subtables remain.

Written by the indexing model from the issue text.