Add several packet validations.

Frameworks/MQTTnet.NetStandard/Adapter/MqttChannelAdapter.cs

@@ -137,7 +137,7 @@ namespace MQTTnet.Adapter
             
             if (header.BodyLength == 0)
             {
-                return new ReceivedMqttPacket(header, new MemoryStream(new byte[0], false));
+                return new ReceivedMqttPacket(header, null);
             }
 
             var body = new MemoryStream(header.BodyLength);

Frameworks/MQTTnet.NetStandard/Adapter/ReceivedMqttPacket.cs

@@ -9,7 +9,7 @@ namespace MQTTnet.Adapter
         public ReceivedMqttPacket(MqttPacketHeader header, MemoryStream body)
         {
             Header = header ?? throw new ArgumentNullException(nameof(header));
-            Body = body ?? throw new ArgumentNullException(nameof(body));
+            Body = body;
         }
 
         public MqttPacketHeader Header { get; }