A geometry is empty if it does not have any points. The envelope, boundary, interior, and exterior of an empty geometry are NULL. An empty geometry is always simple and can have Z coordinates or measures. Empty linestrings and multilinestrings have a 0 length. Empty polygons and multipolygons have a 0 area.
The ST_IsEmpty predicate function takes a geometry and returns 1 (TRUE) if the geometry is empty and 0 (FALSE) otherwise. For more information, see ST_IsEmpty.