Fix corner case in MatthewsCorrcoef

[SkafteNicki]

What does this PR do?

Fixes #2742

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?

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

---

📚 Documentation preview 📚: https://torchmetrics--2743.org.readthedocs.build/en/2743/

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 41%. Comparing base (62d9d32) to head (0905f43).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2743     +/-   ##
========================================
- Coverage      69%     41%    -28%     
========================================
  Files         329     329             
  Lines       18073   18077      +4     
========================================
- Hits        12499    7479   -5020     
- Misses       5574   10598   +5024