test: add case for handling empty compaction results in MixCompactionTask

[sunby]

issue: #47076

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.15%. Comparing base (9373518) to head (95028a7).
⚠️ Report is 11 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #47090       +/-   ##
===========================================
+ Coverage   76.46%   82.15%    +5.68%     
===========================================
  Files        2016      598     -1418     
  Lines      325600    96523   -229077     
===========================================
- Hits       248972    79295   -169677     
+ Misses      68670    17176    -51494     
+ Partials     7958       52     -7906     

Components	Coverage Δ
Client	∅ <ø> (∅)
Core	82.95% <99.47%> (+0.05%)	⬆️
Go	∅ <ø> (∅)
see 1421 files with indirect coverage changes

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