Couldn't merge the fonts, because one of the values in a list was empty when it shouldn't have been. ... GPOS.table.LookupList.Lookup[0][0][1].EntryAnchor

With [linefont](https://github.com/dy/linefont) we updated gftools from v0.9.15 to 0.9.33 and it began throwing error:

```
...
File "/Users/div/projects/linefont/venv/lib/python3.11/site-packages/gftools/builder/__init__.py", line 336, in build_variable
output_files = self.run_fontmake(source, args)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/div/projects/linefont/venv/lib/python3.11/site-packages/gftools/builder/__init__.py", line 395, in run_fontmake
FontProject().run_from_designspace(source, **args)
File "/Users/div/projects/linefont/venv/lib/python3.11/site-packages/fontmake/font_project.py", line 1161, in run_from_designspace
raise FontmakeError(
fontmake.errors.FontmakeError: In 'Linefont.designspace': Generating fonts from Designspace failed:

Couldn't merge the fonts, because one of the values in a list was empty when
it shouldn't have been. This happened while performing the following
operation: GPOS.table.LookupList.Lookup[0][0][1].EntryAnchor

The problem is likely to be in Linefont SuperCondensed Thin:
.EntryAnchor==[None, None, None, None, None, None, None, None]
```

If anyone can have a look and diagnose the issue 🙏🏼

Related https://github.com/dy/linefont/issues/9.

@RosaWagner recommended posting it here, because it's a blocker for the linefont.
Can't downgrade gftools because of [another issue](https://github.com/dy/wavefont/pull/57#issuecomment-1643794879).