`magit-git-string': only move point if buffer contains anything

Using a stock configuration, running `magit-status' results in 27 calls
to `magit-git-string', so the less it does, the better.

Even though it doesn't make a measurable difference, there's no point in
moving `point' unless we're sure the buffer contains anything.

Signed-off-by: Pieter Praet <pieter@praet.org>