If passed fields = [ null, ..., ...] or [ ... , null, ...] we fail in Dataset._normalizeRecordsAndFields method because:
1. If null is first element we do not do field generation correctly because typeof(null) is object - ede211646a/src/model.js (L108)
2. We call toString on field names which fails for null ede211646a/src/model.js (L113)