Never fill None for quality changes profile

We want it to be the empty profile instead. This gets passed on all the way to _replaceQualityOrQualityChangesInStack where the metadata is requested.

Contributes to issue CURA-3301.

cura/Settings/MachineManager.py

@@ -929,6 +929,8 @@ class MachineManager(QObject):
                 quality_changes = quality_changes_list[0]
             else:
                 quality_changes = global_quality_changes
+            if not quality_changes:
+                quality_changes = self._empty_quality_changes_container
 
             material = stack.material
             quality = quality_manager.findQualityByQualityType(quality_type, global_machine_definition, [material])