mqtt: do not log reason_codes if there is none

Ticket: #5167

rust/src/mqtt/logger.rs

@@ -233,12 +233,14 @@ fn log_mqtt(tx: &MQTTTransaction, flags: u32, js: &mut JsonBuilder) -> Result<()
                 log_mqtt_header(js, &msg.header)?;
                 js.set_uint("message_id", unsuback.message_id as u64)?;
                 if let Some(codes) = &unsuback.reason_codes {
+                    if codes.len() > 0 {
                         js.open_array("reason_codes")?;
                         for t in codes {
                             js.append_uint(*t as u64)?;
                         }
                         js.close()?; // reason_codes
                     }
+                }
                 js.close()?; // unsuback
             }
             MQTTOperation::PINGREQ => {