Don't crash if machine_extruder_trains metadata is missing

This old function is only necessary for upgrading from before v3.4. Best not let it crash in any other case, even if that would sometimes make very old machine instances corrupt if I made a mistake in thinking here.

Fixes Sentry issue CURA-3XG.

cura/Settings/ExtruderManager.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2020 Ultimaker B.V.
+# Copyright (c) 2022 Ultimaker B.V.
 # Cura is released under the terms of the LGPLv3 or higher.
 
 from PyQt6.QtCore import pyqtSignal, pyqtProperty, QObject, QVariant  # For communicating data and events to Qt.
@@ -382,7 +382,10 @@ class ExtruderManager(QObject):
     # "fdmextruder". We need to check a machine here so its extruder definition is correct according to this.
     def fixSingleExtrusionMachineExtruderDefinition(self, global_stack: "GlobalStack") -> None:
         container_registry = ContainerRegistry.getInstance()
-        expected_extruder_definition_0_id = global_stack.getMetaDataEntry("machine_extruder_trains")["0"]
+        expected_extruder_stack = global_stack.getMetadataEntry("machine_extruder_trains")
+        if expected_extruder_stack is None:
+            return
+        expected_extruder_definition_0_id = expected_extruder_stack["0"]
         try:
             extruder_stack_0 = global_stack.extruderList[0]
         except IndexError: