ENH:Clear the value of the previous nozzle type

jira:[for nozzle check]

Change-Id: I9a932b833fb07de6cb0a91abe6372b0e91f273f1

src/slic3r/GUI/DeviceManager.cpp

@@ -3560,6 +3560,9 @@ int MachineObject::parse_json(std::string payload, bool key_field_only)
                                     }
                                 }
                             }
+                            else {
+                                nozzle_type = "";
+                            }
                         }
                         catch (...) {
                             ;