Fix corner case of calibration error

[SkafteNicki]

What does this PR do?

Fixes #1646
Happens when a bin has zero confidence. Added code from issue as test.

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

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[codecov]

Codecov Report

Merging #1648 (f1a50a5) into master (685391d) will decrease coverage by 38%.
The diff coverage is 100%.

❗ Current head f1a50a5 differs from pull request most recent head eb9e1cc. Consider uploading reports for the commit eb9e1cc to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1648     +/-   ##
========================================
- Coverage      89%     51%    -38%     
========================================
  Files         228     228             
  Lines       12363   12363             
========================================
- Hits        10945    6304   -4641     
- Misses       1418    6059   +4641