fix edge case in AP_begin_indices

[AurelienJaquier]

Description

Fixes issue #396 .
Since refactoring in PR #334, a feature is not computed when one of its dependency cannot be resolved. This however raised a new edge case: When there is only one spike, and the min_AHP_indices is empty / None (when the only value would be at stim_end, and is thus removed), then one dependency is empty, but the feature could still be computed. To make the feature compute nonetheless in this case, we can catch the 'dependency is None' error before the feature computation stage.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 91.84%. Comparing base (2cddd26) to head (32fc631).
Report is 19 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #397      +/-   ##
==========================================
+ Coverage   91.73%   91.84%   +0.11%     
==========================================
  Files          36       37       +1     
  Lines        6278     7113     +835     
  Branches     2033     2280     +247     
==========================================
+ Hits         5759     6533     +774     
- Misses        266      301      +35     
- Partials      253      279      +26     

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