‼️ Make need fields added by services nullable and default to null

[chrisjsewell]

Rather than non-nullable and default to empty string.

Note, this could be breaking, for need filters that assume a service field value is always a string.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.80%. Comparing base (4e10030) to head (e4620bf).
⚠️ Report is 238 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1644      +/-   ##
==========================================
+ Coverage   86.87%   88.80%   +1.92%     
==========================================
  Files          56       70      +14     
  Lines        6532     9986    +3454     
==========================================
+ Hits         5675     8868    +3193     
- Misses        857     1118     +261     

Flag	Coverage Δ
pytests	88.80% <100.00%> (+1.92%)	⬆️

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