fix(backend-github): don't create new commits on empty diff when rebasing

[erezrokah]

Empty commits could have been created before #3306 was merged and published.

This fixes rebasing of entries created with empty commits.

[erquhart]

Should we add some test coverage for this case?

_{Sent with GitHawk}

[erezrokah]

Should we add some test coverage for this case?

This is quite an unusual case reflected by another bug.
I've been meaning to add edge cases scenarios to our e2e testing (e.g. out of sync forks, larger that 250 commits diffs). I'll add that to the list.

[erezrokah]

Context #3373