hooks: avoid warnings when no files are checked

`cros lint` shows a warning, so return early when there are no files.

BUG=b:213235948
TEST=CQ passes

Change-Id: Id1320d44faa1d9deb604c04a16d7b9f945bea8b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/3529579
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Allen Webb <allenwebb@google.com>
Commit-Queue: Allen Webb <allenwebb@google.com>
1 file changed