branch: do not fail a no-op --edit-desc

Imagine running "git branch --edit-description" while on a branch
without the branch description, and then exit the editor after
emptying the edit buffer, which is the way to tell the command that
you changed your mind and you do not want the description after all.

The command should just happily oblige, adding no branch description
for the current branch, and exit successfully.  But it fails to do
so:

    $ git init -b main
    $ git commit --allow-empty -m commit
    $ GIT_EDITOR=: git branch --edit-description
    fatal: could not unset 'branch.main.description'

The end result is OK in that the configuration variable does not
exist in the resulting repository, but we should do better.  If we
know we didn't have a description, and if we are asked not to have a
description by the editor, we can just return doing nothing.

This of course introduces TOCTOU.  If you add a branch description
to the same branch from another window, while you had the editor
open to edit the description, and then exit the editor without
writing anything there, we'd end up not removing the description you
added in the other window.  But you are fooling yourself in your own
repository at that point, and if it hurts, you'd be better off not
doing so ;-).

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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