Fix Error 500 when viewing a merge request with a deleted source project

Merged Stan Hu requested to merge sh-fix-issue-33011 into master

If the merge request can be rebased based on the history of the commits, users attempting to view the merge request would hit an Error 500. This occurred because user_can_push_to_source_branch? would be called on a non-existent project.

Closes gitlab-org/gitlab-ce#33011

Edited by Stan Hu

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