Fix corner case for R2Score

[SkafteNicki]

What does this PR do?

Fixes #1573

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 🙃

[codecov]

Codecov Report

Merging #1576 (28b8bd8) into master (f9514c0) will decrease coverage by 50%.
The diff coverage is 100%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1576     +/-   ##
========================================
- Coverage      88%     38%    -50%     
========================================
  Files         223     223             
  Lines       11656   11661      +5     
========================================
- Hits        10310    4488   -5822     
- Misses       1346    7173   +5827