Fix empty buffer handling in TlsHandler

[maksimkim]

Problem: promise associated with writing empty buffer to TlsHandler never completes because EmptyByteBuffer doesn't write anything on .ReadBytes call to passed stream:

SpanNetty/src/DotNetty.Handlers/Tls/TlsHandler.Writer.cs

Line 171 in 9f31f09

_ = buf.ReadBytes(_sslStream, readableBytes); // this leads to FinishWrap being called 0+ times

Minimal repro: https://github.com/maksimkim/DotNetty/blob/2a35ea6df4b57f33997cb00702a1c712d6a17c6b/examples/HttpClient/Program.cs#L60

await channel.WriteAndFlushAsync(EmptyLastHttpContent.Default) never completes.

Solution: special case handling for empty buffer

[maksimkim]

@cuteant , the change breaks existing TlsHandler tests when checking EmbeddedChannel.Finish():

SpanNetty/test/DotNetty.Handlers.Tests/TlsHandlerTest.cs

Line 126 in 9f31f09

Assert.False(ch.Finish());

It expects channel not to have any outgoing messages but the Finish() itself causes TlsHandler to write another Unpooled.Empty to the EmbeddedChannel:

SpanNetty/src/DotNetty.Handlers/Tls/TlsHandler.Writer.cs

Line 105 in 9f31f09

_pendingUnencryptedWrites.Add(Unpooled.Empty, promise);

Hence this validation always fails. What's going to be your recommendation on proper fix for the tests?

[maksimkim]

@cuteant , also the regression is introduced in this PR: #65
Prior to that the promise for empty buffer write was completed in place without writing to channel:

SpanNetty/src/DotNetty.Handlers/Tls/TlsHandler.Writer.cs

Line 161 in 34a24e3

_ = promise.TryComplete();

I wonder what is the original netty behavior in this situation?