Avoid numpy scalar deprecation warnings in __setitem__

[dopplershift]

NumPy as of 1.25 deprecated automatically converting any "scalar" with non-zero number of dimensions to a float value. Therefore, we should ensure our values have ndim == 0 before passing to math.isnan()

• Closes # (insert issue number)
• Executed pre-commit run --all-files with no errors
• The change is fully covered by automated unit tests
• Documented in docs/ as appropriate
• Added an entry to the CHANGES file

[codspeed-hq]

CodSpeed Performance Report

Merging #1880 will not alter performance

_{Comparing dopplershift:avoid-warning (5bc72ee) with master (fc1aeba)}

Summary

✅ 439 untouched benchmarks

[hgrecco]

Thanks!