Returns a polyline containing the convex hull of this polyline's points.
The output polyline begins with the first point of this polyline that is on the hull. The convex hull points then follow in clockwise order.
The minimum number of points is reported. This means that collinear points lying in the middle of hull edges are not reported. If a group of coincident points is encountered, only the first point (in order of the original polyline) is reported.
If the polyline contains no points,
null is returned.