git_sha1_gen: create empty file in fallback path

I missed this part in my conversion, the old stream redirection meant
the file was always created.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103496
Fixes: 7088622e5fb506b64c90 "buildsys: move file regeneration logic to
       the script itself"
Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
1 file changed