Mark PrinterOutputDevice without printer type as invalid

Since self._printer_type was defined as an empty string and the setter only accepts strings, it could never be None

cura/PrinterOutput/ConfigurationModel.py

@@ -54,7 +54,7 @@ class ConfigurationModel(QObject):
         for configuration in self._extruder_configurations:
             if configuration is None:
                 return False
-        return self._printer_type is not None
+        return self._printer_type != ""
 
     def __str__(self):
         message_chunks = []