update(remap_array): trap for None type idomain

[jlarsen-usgs]

• reset mfarray value for idomain to constant 1 if idomain is unspecified

closes #2027

[codecov]

Codecov Report

Merging #2034 (2ad7b71) into develop (9c6c234) will decrease coverage by 0.1%.
Report is 1 commits behind head on develop.
The diff coverage is 100.0%.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2034     +/-   ##
=========================================
- Coverage     72.9%   72.9%   -0.1%     
=========================================
  Files          259     259             
  Lines        59047   59049      +2     
=========================================
- Hits         43071   43066      -5     
- Misses       15976   15983      +7     

Files	Coverage Δ
flopy/mf6/utils/model_splitter.py	70.0% <100.0%> (+<0.1%)	⬆️

... and 1 file with indirect coverage changes