Empty messages are "randomly" skipped

[vadrsa]

The following code in WebSocketServerTransport.cs results in some of the empty messages being skipped.

// Do a 0 byte read so that idle connections don't allocate a buffer when waiting for a read
var result = await socket.ReceiveAsync(Memory<byte>.Empty, token);

if (result.MessageType == WebSocketMessageType.Close)
{
    _gracefulClose = true;
    return;
}

Steps to reproduce

1. Run an echo server
2. Send 2 empty messages
3. One will be echoes back, one will not