io/json: refactor `load_ndjson`
io/json: refactor `load_ndjson`

Refactor the awkward conditional in `load_ndjson` to explicitly
skip lines that are empty when requested.

Co-authored-by: Thomas Sibley <c14fd52fbd144b728fef6fa21a04b4bbfd6a668e@fredhutch.org>

nextstrain/augur,nextstrain/augur,nextstrain/augur