ISI_CV: return None when None

[AurelienJaquier]

Description

Please include a summary of the change.
By default the change will increment the patch version.
To increment major or minor changes add #major or #minor to the PR description.

Checklist:

• Unit tests are added to cover the changes (skip if not applicable).
• The changes are mentioned in the documentation (skip if not applicable).
• CHANGELOG file is updated (skip if not applicable).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 91.86%. Comparing base (bffef33) to head (171f25c).
Report is 3 commits behind head on master.

Files	Patch %	Lines
efel/pyfeatures/isi.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #373       +/-   ##
===========================================
+ Coverage   68.49%   91.86%   +23.37%     
===========================================
  Files          36       36               
  Lines        6446     6258      -188     
  Branches     2087     2024       -63     
===========================================
+ Hits         4415     5749     +1334     
+ Misses        275      258       -17     
+ Partials     1756      251     -1505     

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[darshanmandge]

Works with an example which had error similar to that in #372