Better fallbacks if MachineInstance missing profiles

They now fallback to "empty" so that it can still construct a valid
instance, albeit with a missing profile (such as material), if some
profile is missing from the original machine instance file.

Contributes to issue CURA-844.

plugins/VersionUpgrade/VersionUpgrade21to22/MachineInstance.py

@@ -47,11 +47,11 @@ class MachineInstance:
             raise UM.VersionUpgrade.InvalidVersionException("The version of this machine instance is wrong. It must be 1.")
 
         self._type_name = config.get("general", "type")
-        self._variant_name = config.get("general", "variant", fallback = None)
-        self._name = config.get("general", "name")
+        self._variant_name = config.get("general", "variant", fallback = "empty")
+        self._name = config.get("general", "name", fallback = "")
         self._key = config.get("general", "key", fallback = None)
-        self._active_profile_name = config.get("general", "active_profile", fallback = None)
-        self._active_material_name = config.get("general", "material", fallback = None)
+        self._active_profile_name = config.get("general", "active_profile", fallback = "empty")
+        self._active_material_name = config.get("general", "material", fallback = "empty")
 
         self._machine_setting_overrides = {}
         for key, value in config["machine_settings"].items():