[release/8.0-staging] remove extra assert from WinHttp handler

[github-actions]

Backport of #93984 to release/8.0-staging

/cc @ManickaP @wfurt

Customer Impact

• Customer reported
• Found internally

Discovered in our CI, the issue is fixed since .NET 9. The problem is that from a certain Windows version, WinHTTP started calling the affected callback sometimes twice (we suspected it came with TLS 1.3). So the change now checks for the second call by checking whether RequestHandle is null or not. Technically the change just swaps assert for if not null.

Note that I tried changes without product code impact without success in #112780.

Regression

• Yes
• No

Testing

CI, confirming that WinHttpHandler tests do not crash.

Risk

Low. Firstly, this fix is out in 9.0 and 10.0. Secondly, it's very small change replacing assert with if not null.

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