misc: use nullable getters in ConstraintContext

[superlopuh]

Paring this PR down until it's clear that ScopedDicts are what we want to use for constraint contexts, reducing changes to just getters being nullable.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.34%. Comparing base (150b84b) to head (9318fdf).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3503      +/-   ##
==========================================
+ Coverage   90.32%   90.34%   +0.01%     
==========================================
  Files         464      464              
  Lines       58067    58055      -12     
  Branches     5561     5550      -11     
==========================================
- Hits        52451    52448       -3     
+ Misses       4187     4179       -8     
+ Partials     1429     1428       -1     

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[alexarice]

Great change

[math-fehr]

Uncontroversial