The `selected_target_project` helper method is used for actions where
`target_project_id` is a top-level parameter, not nested under the
`merge_request` hash.

Even though that logic was wrong, if the target project wasn't a fork,
or was a fork of an active project, this would work through
coincidence (although it might check the wrong value for
`approvals_before_merge`).

If a project was a fork of a deleted project, however, then
`selected_target_project` would be `nil`, causing a 500 error. Ensure
that there is always a target project to compare to.