Bufix/220 null ref when bool explicitly set

[jamie-taylor-rjj]

Rationale for this PR

This PR attempts to fix the bug reported in #220 by validating the state of the middleware configuration using a naive approach: is there a non-null POCO for each flag set to true?

Whilst this PR achieves the validation, it does not stop the situation from happening in the first place as this would be an API surface change and we'll push that to the next major version.

PR Checklist

Feel free to either check the following items (by place an x inside of the square brackets) or by replacing the square brackets with a relevant emoji from the following list:

• ✅ to indicate that you have checked something off
• ❎ to indicate that you haven't checked something off
• ❓ to indicate that something might not be relevant (writing tests for documentation changes, for instance)

Essential

These items are essential and must be completed for each commit. If they are not completed, the PR may not be accepted.

• [ ✅ ] I have added tests to the OwaspHeaders.Core.Tests project
• [ ✅ ] I have run the dotnet-format command and fixed any .editorconfig issues
• [ ✅ ] I have ensured that the code coverage has not dropped below 65%
• [ ✅ ] I have increased the version number in OwaspHeaders.Core.csproj (only relevant for code changes)
• [ ✅ ] I have updated the changelog in the root of the repository
• [ ✅ ] I have avoided using primary constructors in the example project (see README for details)

Note

The changelog in the docs/ directory will be updated automatically when PRs are merged into main

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
OwaspHeaders.Core	90%	75%	511	✔
OwaspHeaders.Core	90%	75%	511	✔
OwaspHeaders.Core	90%	75%	511	✔
Summary	90% (3036 / 3372)	75% (840 / 1122)	1533	✔