Fix #46123 - make the subProtocol nullable for HttpListenerContext::AcceptWebSocketAsync

[euantorano]

Fixes #46123. This makes the subProtocol nullable when accepting web sockets.

A null subProtocol is valid and means no subprotocol, whereas an empty string is not valid so there needs to be a distinction between them.

I hope I've covered everything that needs doing here, but as this is my first contribution please let me know if there's anything I've missed.

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Issue Details

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Fixes #46123. This makes the subProtocol nullable when accepting web sockets.

A null subProtocol is valid and means no subprotocol, whereas an empty string is not valid so there needs to be a distinction between them.

I hope I've covered everything that needs doing here, but as this is my first contribution please let me know if there's anything I've missed.

Author:	euantorano
Assignees:	-
Labels:	area-System.Net, new-api-needs-documentation
Milestone:	-

[dnfadmin]

All CLA requirements met.

[antonfirsov]

Looks like we even have the tests in HttpListenerContextTests.cs to cover the subProtocol=null cases, it's just the test project has not been updated to use nullable references.

Should be good to merge if #46123 gets the API approval.

[antonfirsov]

/azp run runtime

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[antonfirsov]

/azp run runtime-libraries-coreclr outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[stephentoub]

@jeffhandley, I don't remember how/where you wanted to track nullability changes...

[jeffhandley]

@jeffhandley, I don't remember how/where you wanted to track nullability changes...

Here's the running issue: dotnet/docs#21202
Thanks!