Fix handling of nullable structs

[martincostello]

• Fix handling of nullable structs.
• Add missing verification tests for the Basic website.
• Update various NuGet packages to their latest versions due to dependabot failing.

Resolves #3013.

[martincostello]

Before fix:

TestStruct: {
  type: object,
  properties: {
    hasValue: {
      type: boolean,
       readOnly: true
    },
    value: {
      $ref: #/components/schemas/TestStruct
    }
  },
  additionalProperties: false
}

After fix:

TestStruct: {
  type: object,
  properties: {
    a: {
      type: integer,
      format: int32
    },
    b: {
      type: integer,
      format: int32
    }
  },
  additionalProperties: false
}

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.61%. Comparing base (30fb944) to head (0c1bfbf).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3015   +/-   ##
=======================================
  Coverage   90.61%   90.61%           
=======================================
  Files          74       74           
  Lines        2962     2962           
  Branches      469      469           
=======================================
  Hits         2684     2684           
  Misses        278      278           

Flag	Coverage Δ
Linux	90.61% <100.00%> (ø)
Windows	90.61% <100.00%> (ø)
macOS	90.61% <100.00%> (ø)

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[mesMess]

MSBuild name fix
Swaggy new regex
Some failing test fixes and cleanup