Cannot distinguish whether the session expiry interval of a DISCONNECT packet is zero or absent

[Y-Sindo]

Describe the bug

The field type of MqttDisconnectPacket.SessionExpiryInterval is uint instead of Nullable<uint>, which means we cannot tell whether the session expiry interval is set or zero. According to the MQTT protocol 5.0 spec, the absence and zero-value of the session expiry interval have different meaning.

To Reproduce

https://github.com/dotnet/MQTTnet/blob/b9683c29436cace6dfa0b00affbe6b5f03e06b5b/Source/MQTTnet/Packets/MqttDisconnectPacket.cs#L30C1-L30C1

Expected behavior

Use long or uint? instead of uint to represent the field