fixing small bug in polygon constructor that causes an exception when an empty list is passed in for the holes.
fixing small bug in polygon constructor that causes an exception when an empty list is passed in for the holes.

pombreda/pysal,google-code-export/pysal,pombreda/pysal,pombreda/pysal,google-code-export/pysal,pombreda/pysal,google-code-export/pysal,google-code-export/pysal