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
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.