Fix corner case when single empty pred is provided to detection IOU metric

[SkafteNicki]

What does this PR do?

Fixes #2778

Before submitting

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

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📚 Documentation preview 📚: https://torchmetrics--2780.org.readthedocs.build/en/2780/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 36%. Comparing base (b1f4db2) to head (89bc170).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2780     +/-   ##
========================================
- Coverage      70%     36%    -34%     
========================================
  Files         336     336             
  Lines       18229   18231      +2     
========================================
- Hits        12705    6569   -6136     
- Misses       5524   11662   +6138