Fix resource_tokens page in credentials

Related to #512215 (closed)

What does this MR do and why?

When a user creates a grAT and the user is deleted, it results in a nil error on the credential inventory page.

This MR fixes this issue by falling back to a Deleted user when a grAT that a deleted user created is referenced.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create a group access token
2. Delete the user you used to create the grAT
3. Wait for the user to be fully deleted by the ::DeleteUserWorker
4. Navigate to /admin -> credentials -> Project and group access tokens
5. See the Deleted user text under the Creator column of the recently created grAT

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