* Handle missing reference values in scorer
Handle missing values in reference doc during scoring where it is
possible to detect an unset state for the attribute. If no reference
docs contain annotation, `None` is returned instead of a score. `spacy
evaluate` displays `-` for missing scores and the missing scores are
saved as `None`/`null` in the metrics.
Attributes without unset states:
* `token.head`: relies on `token.dep` to recognize unset values
* `doc.cats`: unable to handle missing annotation
Additional changes:
* add optional `has_annotation` check to `score_scans` to replace
`doc.sents` hack
* update `score_token_attr_per_feat` to handle missing and empty morph
representations
* fix bug in `Doc.has_annotation` for normalization of `IS_SENT_START`
vs. `SENT_START`
* Fix import
* Update return types
a4b32b9552