GeometryCollection(g1, g2, ...)

Constructs a GeometryCollection.

As of MySQL 5.7.5, GeometryCollection() returns all the proper
geometries contained in the argument even if a nonsupported geometry is
present. Before 5.7.5, if the argument contains a nonsupported
geometry, the return value is NULL.

As of MySQL 5.7.8, GeometryCollection() with no arguments is permitted
as a way to create an empty geometry.

URL: http://dev.mysql.com/doc/refman/5.7/en/gis-mysql-specific-functions.html

