true if the coordinate is valid
null if none found
The default (following the OGC SFS standard) is that this condition is not valid (false).
This does not affect whether Self-Touching Rings disconnecting the polygon interior are considered valid (these are considered to be invalid under the SFS, and many other spatial models as well). This includes "bow-tie" shells, which self-touch at a single point causing the interior to be disconnected, and "C-shaped" holes which self-touch at a single point causing an island to be formed.