git rebase -i does not rebase if all lines are removed

Make a commit on top of master.

git rebase -i origin/master

Remove the commit.

Git prints "Nothing to do" and does not rebase.

Running 'git rebase -i' when there are no local commits has 'noop' in
the first line, and with it the rebase is successful. Why is this
'noop' mandatory?

- Orgad