`zero_regex_matches.length` isn't going to be a good route. Zero is truthy in ruby.

```ruby
puts "foo" if 0
```

I think this logic may be easier to express as "If it is 0[px|em|ex|pt|etc], return "0" the units allowed [are known](https://github.com/mduvall/csscss/blob/3f361f56e8bb6aabd4cf682b10c8fafdbcc47508/lib/csscss/parser/common.rb#L6). You may need to add `%` to that list though.