The only external entry point to the ref_store lookup functions is
get_ref_store(), which ensures that submodule == "" is passed along as
NULL. So ref_store_init() and lookup_ref_store() don't have to handle
submodule being specified as the empty string.
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>