[🚀 Feature]: .NET: Support nullable reference types

[RenderMichael]

Feature and motivation

Annotate the code for nullable reference type support. This would be massively beneficial for clarity and to prevent null reference exceptions, both within Selenium and for users.

Usage example

[github-actions[bot]]

@RenderMichael, thank you for creating this issue. We will troubleshoot it as soon as we can.

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Thank you!

[RenderMichael]

If Selenium is interested in accepting NRT, I'll gladly take up the task

[nvborisenko]

It would be great, and it would be challenging as for you as for us :) Only one request: try keeping PRs small.

[nvborisenko]

@RenderMichael do you mind to join Selenium Slack channel for messaging instantly?