AUTH packets with no body should be allowed

[gordonwang0]

Verification

Per the MQTT spec, AUTH packets with Reason Success are allowed to omit the body:

The Reason Code and Property Length can be omitted if the Reason Code is 0x00 (Success) and there are no Properties. In this case the AUTH has a Remaining Length of 0.

Therefore, the AUTH packet [0xf0, 0x00] is valid, as this has:

• 0xf0 : AUTH header
• 0x00 : Remaining length = 0

Describe the bug

MQTTnet doesn't accept these packets. It throws an exception when trying to parse these packets:

MQTTnet.Exceptions.MqttProtocolViolationException: Data from the body is required but not present.
   at MQTTnet.Formatter.V5.MqttV5PacketDecoder.ThrowIfBodyIsEmpty(ArraySegment`1 body)
   at MQTTnet.Formatter.V5.MqttV5PacketDecoder.DecodeAuthPacket(ArraySegment`1 body)
   at MQTTnet.Formatter.V5.MqttV5PacketDecoder.Decode(ReceivedMqttPacket receivedMqttPacket)
   at MQTTnet.Formatter.V5.MqttV5PacketFormatter.Decode(ReceivedMqttPacket receivedMqttPacket)
   at MQTTnet.Formatter.MqttPacketFormatterAdapter.Decode(ReceivedMqttPacket receivedMqttPacket)
   at MQTTnet.Adapter.MqttChannelAdapter.ReceivePacketAsync(CancellationToken cancellationToken)
   at MQTTnet.Client.MqttClient.Receive(CancellationToken cancellationToken)
   at MQTTnet.Client.MqttClient.ReceivePacketsLoop(CancellationToken cancellationToken)

Which component is your bug related to?

• Client

Expected behavior

The AUTH packet [0xf0, 0x02, 0x00, 0x00] works fine. This has:

• 0xf0 - AUTH header
• 0x02 - Remaining length = 2
• 0x00 - Reason Success
• 0x00 - Property length = 0

Per the MQTT spec, the AUTH packets [0xf0, 0x00] and [0xf0, 0x02, 0x00, 0x00] should be treated as equivalent, so both should work with MQTTnet.