The semantics of NULL are well defined for us already in SQL standards. However, in the world of Geometry / Geography we have another entity which is not quite NULL nor quite fully defined: the empty geometry. Empty geometries can be created with things like intersection calls that have have no intersection, e.g. ST_Intersect('POINT(0 0)', 'POINT(1 1)') == POINT EMPTY. So, in the presence of empty, how should functions behave?

 * ST_Union(geometry, empty) == geometry
 * ST_Union(empty, empty) == empty
 * ST_Difference(geometry, empty) == geometry
 * ST_Difference(empty, geometry) == empty
 * ST_Distance(geometry, empty) == NULL
 * ST_DWithin(geometry, empty, tolerance) == FALSE
 * ST_Contains(geometry, empty) == FALSE
 * ??? ST_Contains(empty, empty) == FALSE
 * ST_Intersects(geometry, empty) == FALSE
 * ??? ST_Intersects(empty, empty) == FALSE
 * ??? ST_Disjoint(empty, empty) == FALSE
 * ??? ST_Disjoint(geometry, empty) == FALSE
 * ST_IsSimple(empty) == TRUE
 * ST_IsValid(empty) == TRUE