AutoroutePartIndex from throwing exception when content definition is not found

[MikeAlhayek]

Describe the bug

When Autoroute feature is enabled, and one is updating content item where content-type is missing, AutoroutePartIndex from throwing exception when content definition is not found

To Reproduce

Steps to reproduce the behavior:

1. Enable Autoroute
2. Create Content Type called "Test"
3. Create content item of "Test" called "Test Item"
4. Delete the content type "Test"
5. Edit the "Test Item"
6. Click Save

Expected behavior

AutoroutePartIndex should not throw an exception