Fix StraightSkeleton test for invalid types
The current test was really just a duplication of another test. Passing invalid input types (points/lines) to StraightSkeleton returns an empty multilinestring (I hadn't changed this behaviour)
The current test was really just a duplication of another test. Passing invalid input types (points/lines) to StraightSkeleton returns an empty multilinestring (I hadn't changed this behaviour)