Do not allow rsync destination to be empty

[stephen-riggs]

The rsyncer destination determination was very out-of-date and potentially caused a problem on Krios 3.
This updates the logic and adds some tests.

[codecov]

Codecov Report

❌ Patch coverage is 78.94737% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.80%. Comparing base (cdbf693) to head (5ccc085).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #718      +/-   ##
==========================================
+ Coverage   42.49%   42.80%   +0.30%     
==========================================
  Files          94       94              
  Lines       10347    10341       -6     
  Branches     1346     1345       -1     
==========================================
+ Hits         4397     4426      +29     
+ Misses       5733     5696      -37     
- Partials      217      219       +2     

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[tieneupin]

Thanks for doing this. Looks good to me, and seems to have worked on all our supported workflows so far.