WL#2058: GIS: Make spatial keys null-safe

Affects: Server-7.0   —   Status: Assigned   —   Priority: Medium

Now spatial keys don't accept NULL values, so SPATIAL field usually has to be
NOT NULL.
But for some tasks it'd be suitable to have nulls in the spatial columns.
Techinically, I think easiest solution is to add the separate list for NULL-s to
the RTree where we put not-nulls.