DM-48589: Fix deserialization of nulls from Arrow Timespan

[dhirving]

Fix a bug where null values in nullable arrow Timespan columns would raise an exception when deserializing an Arrow Table to python.

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes
• (if changing dimensions.yaml) make a copy of dimensions.yaml in configs/old_dimensions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.47%. Comparing base (e060d3e) to head (abc812b).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1153   +/-   ##
=======================================
  Coverage   89.47%   89.47%           
=======================================
  Files         367      367           
  Lines       49190    49198    +8     
  Branches     5959     5960    +1     
=======================================
+ Hits        44014    44022    +8     
  Misses       3757     3757           
  Partials     1419     1419           

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