Returns the maximum Y coordinate value of the geometry-expression. This is computed by
comparing the Y attribute of all points in the geometry.
Note that in round-Earth model, minimum corresponds to southernmost point of the geometry-expression (which may not be one of the points defining the geometry) and maximum corresponds to the northernmost point of the geometry-expression.
If the geometry-expression is an empty geometry (ST_IsEmpty()=1), then this method returns NULL.