Resolve ConvertPayloadToString resulting in ArgumentNullException (#1745)

Resolve breaking change where previously ConvertPayloadToString returned null if the payload was null

Author: ramonsmits

Source/MQTTnet/MqttApplicationMessageExtensions.cs

@@ -4,6 +4,7 @@
 
 using System;
 using System.Text;
+using MQTTnet.Internal;
 
 namespace MQTTnet
 {
@@ -16,6 +17,11 @@ public static string ConvertPayloadToString(this MqttApplicationMessage applicat
  throw new ArgumentNullException(nameof(applicationMessage));
  }
 
+ if(applicationMessage.PayloadSegment == EmptyBuffer.ArraySegment)
+ {
+ return null;
+ }
+
  var payloadSegment = applicationMessage.PayloadSegment;
  return Encoding.UTF8.GetString(payloadSegment.Array, payloadSegment.Offset, payloadSegment.Count);
  }