events['timestamp'] is nan when data has bmi3d but no ecube data

When there is a `bmi3d_clock `but no `sync_clock`, ideally `events['timestamp']` should be set to `events['timestamp_bmi3d']`. But in `prepare_bmi3d_v1`, it continues to get set to `events['timestamp_sync']`. In entries without sync data, `events['timestamp_sync']` is all nan. The tabulate functions use `events['timestamp']`, so this field shouldn't be left as nan.