Rework validation of FoldMask to not fail on tail nans

[d-a-bunin]

Before submitting (must do checklist)

• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

Proposed Changes

See #531.

Closing issues

Closes #531.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.35%. Comparing base (f527fdb) to head (b99b730).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #536      +/-   ##
==========================================
- Coverage   90.36%   90.35%   -0.01%     
==========================================
  Files         256      256              
  Lines       17303    17299       -4     
==========================================
- Hits        15635    15631       -4     
  Misses       1668     1668              

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