Handle "_" value for token pos in conllu data

[apjanco]

Description

If a conllu file has no data for a token's part of speech, conllu_sentence_to_doc() throws an error that '_' is not a valid upos value.
To allow tokens with no pos data, this PR changes the "_" to "" to allow pos assignment during spacy convert from conllu. It's a small change that will help our users with less than complete and polished treebanks.

Types of change

Handles an exception when UD has a upos value of "_".

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[adrianeboyd]

Thanks, this is a good idea, since spacy definitely supports missing values in annotation. Let me just make a short suggestion (you can commit it directly from the comment) that uses the same style as the surrounding code.

[adrianeboyd]

And looking at this again, I don't think the morph _ value is handled correctly here, since this is a valid empty value rather than a missing value, but this would be a separate PR...

Edit: most likely nevermind about this (I've gone through all of this in detail multiple times in the past, since empty vs. missing is always tricky), since adding the morph value "" is converted to the empty morph value "_". It's possible there's no good way to have a missing value in the JSON training data because of this, but CoNLL-U can't represent this distinction either, so assuming that it's empty is probably fine for this converter in practice.

[apjanco]

Thank you for all your help and style suggestion. My first PR with spaCy😊