no default ACLs = OG compatibility

Hi,

We use Organic Groups access control on our site. Some time ago, we realized the need for "per-user per-node" ACL, in addition to OG. Typical use case is to allow temporary view/edit operation for protected node for one or two people, not included in a current OG group. We evaluated Nodeaccess module, but its default behavior should be a bit different in our case. We required no access control at all (NULL return), if there are no Nodeaccess grants defined, so "public" OG node stays "public" ("all" realm is created by node module). So, I've made a patch to allow OG-compatible behavior (attached) - there's one extra checkbox in Nodeaccess settings to allow "return NULL".

Thanks,
Dmitry