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 :white_check_mark:

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

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Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/domaindrivendev/Swashbuckle.AspNetCore/pull/3119/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Richard+Morris) | Coverage Δ | | |---|---|---| | [Linux](https://app.codecov.io/gh/domaindrivendev/Swashbuckle.AspNetCore/pull/3119/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Richard+Morris) | `91.60% <100.00%> (+0.16%)` | :arrow_up: | | [Windows](https://app.codecov.io/gh/domaindrivendev/Swashbuckle.AspNetCore/pull/3119/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Richard+Morris) | `89.37% <100.00%> (-2.07%)` | :arrow_down: | | [macOS](https://app.codecov.io/gh/domaindrivendev/Swashbuckle.AspNetCore/pull/3119/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Richard+Morris) | `91.60% <100.00%> (+0.16%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Richard+Morris#carryforward-flags-in-the-pull-request-comment) to find out more.

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