Allow HttpSys zero-byte reads #41305

[Tratcher]

Contributes to #41305

When passed a zero-byte buffer Http.Sys returns ERROR_MORE_DATA. We need to handle this specially. I also had to change the argument validation.

Perf impact:

• zero-byte reads to Http.Sys always complete async, even when data is available and a normal read could complete sync. This adds latency to the request.
• Setup: Upload a 20mb request body using a 16k read buffer.
  • No zero-byte read: 65ms
  • With zero-byte reads: 105ms.