Y26-011 - PoolReleased dead letters

[BenTopping]

Closes #sanger/General-Backlog-Items#691

Changes proposed in this pull request

• Prevents creation_requests causing an error if none exist on multiplexed_library_tube

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.35%. Comparing base (8fc8826) to head (587e9eb).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5542      +/-   ##
===========================================
- Coverage    87.36%   87.35%   -0.01%     
===========================================
  Files         1463     1463              
  Lines        33005    33005              
  Branches      3467     3468       +1     
===========================================
- Hits         28834    28832       -2     
- Misses        4150     4152       +2     
  Partials        21       21              

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