Checking required fields doesn't work properly

[Heliozoa]

Hi,
Thanks for your work on genanki-rs.

I noticed in my project that with the following qfmt

<div id=sentence>
    {{furigana:sentence}}
</div>

the Model::req function determines that all of the fields of the model

(

        vec![
            Field::new("id"),
            Field::new("count"),
            Field::new("word_id"),
            Field::new("sentence_id"),
            Field::new("sentence"),
            Field::new("word"),
            Field::new("translation"),
            Field::new("kanji"),
            Field::new("generated_at"),
        ]

)

are required, which is not the case. It will then skip creating cards for any note where any field is empty. I don't really understand the intended logic of req so I'm not sure where the issue is exactly.

[njam]

I agree the logic to detect required fields is wrong.
Here's a PR with a fix: #22
I've implemented the logic based on the Python library's code.