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: 7088622e5f "buildsys: move file regeneration logic to
       the script itself"
Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>

bin/git_sha1_gen.py

@@ -45,3 +45,5 @@ if git_sha1:
                     quit()
         with open(args.output, 'w') as git_sha1_h:
             git_sha1_h.write(new_sha1)
+else:
+    open(args.output, 'w').close()