allow datatransfer vendor id empty

EVCB_OCPP.Packet/Messages/Core/DataTransferRequest.cs

@@ -19,7 +19,6 @@ namespace EVCB_OCPP.Packet.Messages.Core
         [JsonIgnore]
         public string Action { set; get; }
 
-        [Required]
         [MaxLength(255)]
         public string vendorId { set; get; }