Sample collection date and time cannot only have one null

When creating a sample object [from DB](https://github.com/biocore/microsetta-private-api/blob/07a57578d63dc449b4b1e85cb80592a0a24767ea/microsetta_private_api/model/sample.py#L54), the code only creates the `datetime_collected` field when both date and time are non-null. In some situations there might be barcodes that only has the sampled date or time but not both, so that the db information is not accurately represented in downstream flows (e.g. finding sample summaries). @cassidysymons