Fixes #20912: Clear ModuleBay parent when module assignment removed

When a ModuleBay's module field is cleared (returning it to device-level),
the parent field was not being updated, leaving stale MPTT tree data.

The save() method only set parent when module was present, but failed to
clear it when module was removed. Now explicitly sets parent to None when
module is cleared.

netbox/dcim/models/device_components.py

@@ -1222,6 +1222,8 @@ class ModuleBay(ModularComponentModel, TrackingModelMixin, MPTTModel):
     def save(self, *args, **kwargs):
         if self.module:
             self.parent = self.module.module_bay
+        else:
+            self.parent = None
         super().save(*args, **kwargs)