Allow empty input in TransferReadTags

[kachulis]

TransferReadTags will throw an exception if the unaligned bam has no read. This pr will allow the unaligned bam to have 0 reads, as long as the aligned bam also has 0 reads, which will result in an output bam which is just a copy of the aligned bam, with 0 reads.

[codecov]

Codecov Report

Merging #8198 (7608bd2) into master (d1a4f94) will decrease coverage by 0.007%.
The diff coverage is 0.000%.

❗ Current head 7608bd2 differs from pull request most recent head e460b51. Consider uploading reports for the commit e460b51 to get more accurate results

Additional details and impacted files

@@               Coverage Diff               @@
##              master     #8198       +/-   ##
===============================================
- Coverage     86.658%   86.651%   -0.007%     
- Complexity     39024     39155      +131     
===============================================
  Files           2337      2346        +9     
  Lines         182961    183623      +662     
  Branches       20079     20148       +69     
===============================================
+ Hits          158550    159111      +561     
- Misses         17365     17449       +84     
- Partials        7046      7063       +17     

Impacted Files	Coverage Δ
.../hellbender/tools/walkers/qc/TransferReadTags.java	72.549% <0.000%> (-2.961%)	⬇️
...lbender/tools/sv/cluster/CanonicalSVCollapser.java	80.635% <0.000%> (-6.836%)	⬇️
...roadinstitute/hellbender/tools/sv/SVTestUtils.java	90.274% <0.000%> (-6.426%)	⬇️
...ellbender/tools/sv/cluster/CanonicalSVLinkage.java	83.146% <0.000%> (-6.043%)	⬇️
...stitute/hellbender/tools/sv/SVCallRecordUtils.java	79.167% <0.000%> (-4.518%)	⬇️
...ender/utils/variant/GATKSVVariantContextUtils.java	93.939% <0.000%> (-2.835%)	⬇️
...der/tools/walkers/sv/SVClusterIntegrationTest.java	98.974% <0.000%> (-0.522%)	⬇️
.../hellbender/tools/walkers/sv/SVAnnotateEngine.java	89.434% <0.000%> (-0.347%)	⬇️
...stitute/hellbender/tools/walkers/sv/SVCluster.java	89.773% <0.000%> (ø)
...titute/hellbender/utils/GenotypeUtilsUnitTest.java	94.624% <0.000%> (ø)
... and 24 more

[tmelman]

Add test where 1 file has 0 and 1 file has x reads to see that it fails with UserException; add test where both have 0 reads to see that warning is raised and output has 0 reads.

[kachulis]

@tmelman I have added tests, so this is ready for another round of review.

[tmelman]

LGTM