Fixes deserialization exception when message body is empty byte array. (#1087)

src/DotNetCore.CAP/Serialization/ISerializer.JsonUtf8.cs

@@ -36,7 +36,7 @@ namespace DotNetCore.CAP.Serialization
 
         public Task<Message> DeserializeAsync(TransportMessage transportMessage, Type? valueType)
         {
-            if (valueType == null || transportMessage.Body == null)
+            if (valueType == null || transportMessage.Body == null || transportMessage.Body.Length == 0)
             {
                 return Task.FromResult(new Message(transportMessage.Headers, null));
             }