Investigate SslStream.AuthenticateAs*() with empty TargetHost

[wfurt]

This is follow-up on discussion in #38202.
It should be fully optional to specify server name (SNI) when client connects to TLS server.
However, current .NET implementation would create something like "?124" and it would send it on the wire.
This behavior seems to be inherited from Framework but it is not clear why this exists.
If this is limitation of Windows API, that should perhaps live in PAL.

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