feat(python): Enable Null datatype and null values by default in parametric testing

[stinodego]

Null values are a frequent source of bugs, so we should really be testing for them.

Changes

• Enable Null data type by default. Dataframes/Series generated by our strategies may contain a Null column.
• Set allow_null to True by default. This may hit some users, but we should really make the change now while people are updating their parametric tests anyway.
• Setting allow_null=False will exlude the Null data type from the pool.
• Adjust tests to match.

This helped me find a bug, will investigate later:
#16190

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.99%. Comparing base (2e00647) to head (743140c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16192      +/-   ##
==========================================
- Coverage   81.00%   80.99%   -0.01%     
==========================================
  Files        1392     1392              
  Lines      178943   178930      -13     
  Branches     2901     2904       +3     
==========================================
- Hits       144947   144924      -23     
- Misses      33491    33503      +12     
+ Partials      505      503       -2     

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