ETL tasks fail if no target datastreams have data loaded

[kjlippold]

Steps to Reproduce

1. Set up a data loading task with new empty datastreams.

Expected Behavior

HydroServer's internal loader should treat the datastreams as have no earliest phenomenon_end_time

Actual Behavior

The task fails because the loader attempts to find the minimum date of an empty list of end dates.

Any additional comments?

No response