Centroid and InteriorPoint of EMPTY return null

[mdavis]

Centroid and InteriorPoint of * EMPTY return null in the C++ API. They should return POINT EMPTY.

This is fixed in the C API, but the behaviour should be implemented at the C++ level.

[mdavis]

See also #560

[mdavis]

There is an XML test for EMPTY, but the XMLTester has been patched to check for null and create an empty point! So that needs fixing too.

[pramsey]

​https://github.com/libgeos/geos/pull/452

[Paul Ramsey <pramsey@…>]

In 02f9128/git:

Return POINT EMPTY when centroid called on EMPTY input (closes #1088)