Merging bugfix for #200 - np.isnan() handling

[hcpchris]

Merging @ThomasGl's bugfix from #201 into develop as commented here.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.95%. Comparing base (b6925a1) to head (5bc9978).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
highcharts_core/js_literal_functions.py	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #202      +/-   ##
===========================================
- Coverage    87.96%   87.95%   -0.02%     
===========================================
  Files          210      210              
  Lines        31164    31166       +2     
  Branches      2552     2553       +1     
===========================================
- Hits         27413    27411       -2     
- Misses        2791     2793       +2     
- Partials       960      962       +2     

Flag	Coverage Δ
	87.95% <66.66%> (?)

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