Division by zero error

```bash
$ fontquant fa-duotone-900.ttf
Traceback (most recent call last):
File "/Users/roel/.asdf/installs/python/3.10.1/bin/fontquant", line 8, in
sys.exit(cli())
File "/Users/roel/.asdf/installs/python/3.10.1/lib/python3.10/site-packages/fontquant/cli.py", line 60, in cli
quantify(
File "/Users/roel/.asdf/installs/python/3.10.1/lib/python3.10/site-packages/fontquant/__init__.py", line 565, in quantify
return order_dict(base.value(includes, excludes))
File "/Users/roel/.asdf/installs/python/3.10.1/lib/python3.10/site-packages/fontquant/__init__.py", line 425, in value
dictionary[instance.keyword] = instance.value(includes, excludes)
File "/Users/roel/.asdf/installs/python/3.10.1/lib/python3.10/site-packages/fontquant/__init__.py", line 425, in value
dictionary[instance.keyword] = instance.value(includes, excludes)
File "/Users/roel/.asdf/installs/python/3.10.1/lib/python3.10/site-packages/fontquant/casing.py", line 137, in value
dictionary = {"value": self.shape_value(covered_glyphs / eligible_glyphs), "failed": failed_glyphs}
ZeroDivisionError: division by zero
```

It works on different fonts, but this Font Awesome icon font for example fails.