symbols: Fix uneven pairs error on empty []byte
Created by: chrismwendt
Fixes a bug in parseGitDiffOutput
introduced in https://github.com/sourcegraph/sourcegraph/pull/27986
It was affecting a small number of repos on cloud (e.g. kubeovn/kube-ovn between commits b6764e0bc6f5c9effad18a689a275f5894732cda and ba3995d91c3cd9d40882b8755947c7143a3a65).
Test plan
Added a test.