Properly handle average confidence for translation jos with no pretranslations

[Enkidu93]

Fixes #989

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This change is 

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.47%. Comparing base (db04610) to head (c869c36).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #991      +/-   ##
==========================================
- Coverage   65.53%   65.47%   -0.07%     
==========================================
  Files         387      387              
  Lines       21743    21747       +4     
  Branches     2769     2770       +1     
==========================================
- Hits        14249    14238      -11     
- Misses       6454     6472      +18     
+ Partials     1040     1037       -3     

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[ddaspit]

@ddaspit reviewed 2 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on Enkidu93).