Replace np.NaN with np.nan

[ianthomas23]

Following NumPy 2.0 migration guide: https://numpy.org/devdocs/numpy_2_0_migration_guide.html

Thanks to @Hoxbro for highlighting this.

[codecov[bot]]

Codecov Report

Merging #1289 (6572116) into main (b0e9a32) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1289   +/-   ##
=======================================
  Coverage   85.73%   85.73%           
=======================================
  Files          52       52           
  Lines       10870    10870           
=======================================
  Hits         9319     9319           
  Misses       1551     1551           

Files	Coverage Δ
datashader/core.py	88.25% <100.00%> (ø)

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more