Avoid exception checking nullability

[martincostello]

• Do not throw if we cannot determine the nullability of a dictionary. It's better to potentially be wrong, rather than be unable to generate a document at all.
• Clean-up some code analysis suggestions.

Resolves #3070.
Resolves #2793 (as it's the last linked issue).

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.44%. Comparing base (da07a53) to head (2a14a3e).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3119      +/-   ##
==========================================
- Coverage   91.43%   89.44%   -2.00%     
==========================================
  Files          76       97      +21     
  Lines        3119     3523     +404     
  Branches      519      627     +108     
==========================================
+ Hits         2852     3151     +299     
- Misses        267      372     +105     

Flag	Coverage Δ
Linux	91.60% <100.00%> (+0.16%)	⬆️
Windows	89.37% <100.00%> (-2.07%)	⬇️
macOS	91.60% <100.00%> (+0.16%)	⬆️

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[mesMess]

Code cleanup
Changing to concrete types (in-code) over var 👍🏼
Nullability exception check fix