Fix error when normalization mask is empty

[fepegar]

Fixes #655.

Checklist

• I have read the CONTRIBUTING docs and have a developer setup (especially important are pre-commitand pytest)
• Non-breaking change (would not break existing functionality)
• Breaking change (would cause existing functionality to change)
• Tests added or modified to cover the changes
• Integration tests passed locally by running pytest
• In-line docstrings updated
• Documentation updated, tested running make html inside the docs/ folder
• This pull request is ready to be reviewed
• If the PR is ready and there are multiple commits, I have squashed them and force-pushed

[codecov]

Codecov Report

Merging #656 (e4beea2) into master (449bd8f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #656   +/-   ##
=======================================
  Coverage   96.82%   96.83%           
=======================================
  Files         128      128           
  Lines        6646     6657   +11     
=======================================
+ Hits         6435     6446   +11     
  Misses        211      211           

Impacted Files	Coverage Δ
tests/transforms/preprocessing/test_rescale.py	100.00% <100.00%> (ø)
...chio/transforms/preprocessing/intensity/rescale.py	92.85% <100.00%> (+0.75%)	⬆️

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[fepegar]

@all-contributors please add @srg9000 for bug

[allcontributors]

@fepegar

I've put up a pull request to add @srg9000! 🎉