Admin Activities: Show details page gives 500 error if admin has been deleted

1. Get a database admin to create an admin account

2. Log in with that admin account

3. Use that admin account to delete a work (or edit the work's tags, or anything else that would show up on the admin activities page)

4. Get a database admin to delete that admin account

5. Log in as another admin

6. Follow the "Activities" link in the teal admin navigation

7. You'll see a table, and one of the items in that table will say "Admin deleted" in the Admin column. Find that item, and follow the linked date that corresponds to it

What happens
A 500 error

What should happen
The page should load, but it should say "Admin deleted" for "Admin"

Note
This line just needs to say <%= admin_activity.admin.nil? ? ts('Admin deleted') : admin_activity.admin_login %> like the line on the index