gserialized_is_empty cannot handle nested empty cases

[pramsey]

SELECT ST_Distance(
  'MULTILINESTRING(EMPTY,EMPTY)'::geometry::geography, 
  'POINT(0 0)'::geography);

The correct answer is NULL, the actual response is an error, as the geometries slip past the empty guard at the top and into the lwgeom_distance function that returns -1 on error. Stuffing enough empties into the geography circ_tree code seemed to cause enough errors to eventually crash a backend. Not clear why. With the guards that won't happen.

[pramsey]

2.1 at r13698 and trunk at r13700