Fail gracefully on bam with no reads in viral-core::read_utils::minimap2_idxstats(), and/or provide informative error message

[tomkinsc]

Currently, read_utils::minimap2_idxstats() fails when given a bam with a header and read group, but no reads. The guardrails against empty input (i.e. header but not reads) should be improved to fail gracefully in a way that is more helpful for the user to diagnose empty input as the cause of failure.
Example failure: https://job-manager.dsde-prod.broadinstitute.org/jobs/ac7fdc14-db49-4ec4-9b19-e1bdb3e18d00