Validate that per-sample-seqs sets aren't empty

[thermokarst]

Improvement Description
Per-sample sequences should validate that the entire sample set is not empty.

Current Behavior
Validation rules will give the thumbs up to a directory of empty fastq.gz files right now. While technically "okay," this causes problems downstream.

Proposed Behavior
Check that the aggregate dataset is not empty.

References

1. Original forum post