Make JsonlReader path optional (#8396)
Make JsonlReader path optional (#8396)

To avoid config errors during training when `[corpora.pretrain.path]` is
`None` with the default `spacy.JsonlCorpus.v1` reader, make the reader
path optional, similar to `spacy.Corpus.v1`.
explosion/spaCy,honnibal/spaCy,honnibal/spaCy,honnibal/spaCy,explosion/spaCy,explosion/spaCy,explosion/spaCy,explosion/spaCy,honnibal/spaCy,explosion/spaCy