fix: replace mutable empty list by None in Filter class constructor

[ansaminard]

In class Filter, use None as default value for A/B coefficient lists to avoid issues related to the mutable nature of lists

Also:

• added details in class docstring
• replaced == [] tests with len() == 0 tests to avoid not-the-same-shape errors when coefficients are passed as numpy arrays (instead of lists)
• cast to list upon calling the operator to avoid DPF errors when coefficients are passed as numpy arrays (instead of lists)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (cd82963) to head (7fcbc6c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #214   +/-   ##
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  Coverage   100.00%   100.00%           
=========================================
  Files           75        75           
  Lines         4367      4367           
=========================================
  Hits          4367      4367           

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