Resolve ConvertPayloadToString resulting in ArgumentNullException (#1745)

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

Source/MQTTnet/MqttApplicationMessageExtensions.cs

@@ -4,6 +4,7 @@
 
 using System;
 using System.Text;
+using MQTTnet.Internal;
 
 namespace MQTTnet
 {
@@ -16,6 +17,11 @@ namespace MQTTnet
                 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);
         }