Fix 404 error when filtering admin action logs by non-existent target account (#16643)

Fix 404 error when filtering admin action logs by non-existent target account (#16643)

Currently, there is no way for an admin to delete an account record. However,
should that happen in the future, or should an admin mistype an account id,
`/admin/action_logs?target_account_id=<non-existent-account-id>` currently
returns a 404 instead of an empty list.

With this commit, the aforementioned page returns a correct list instead
(potentially non-empty if the account has been manually deleted)