V1.5 Downloads Endpoint Fails If Dataset Has No Submission Date

Example:
See endpoint https://api.neotomadb.org/v1.5/data/Downloads/10495.

Code Reference:
At line 133 of `v1.5/handlers/data_handlers.js`, t.one() expects a result, however, cases exist where datasets do not have a submission date. In these instances, the query fails, no data are returned, and explorer hangs. 