Make robust to non-essential missing fields with clear interpretation

[jeromekelleher]

We should be able to run vcztools view on datasets that don't contain mandatory VCF fields that are often empty and have a clear interpretation when missing.

• filters/filter_id
• ID
• qual?

[tomwhite]

Fixed in #214

[jeromekelleher]

Are there some new rules about when GitHub will see closing notices in commit messages? This doesn't seem to be working as well as it used to.