Ignore an exception thrown by earlier versions of Shapely

In Shapely 1.2.1 polygonize can throw:

  ValueError("Null/empty geometry supports no operations")

... whereas later versions (e.g. 1.2.16) simply yield no items in
the iterator.  With this commit, we catch and ignore any ValueError
thrown by the polygonize method.

As an example, this could be reproduced with:

  >>> from shapely import ops, wkt
  >>> s = wkt.loads('MULTILINESTRING ((40.4761606000000356 40.5267425000000330, 40.4624909000000343 40.5232093000000404))')
  >>> list(ops.polygonize(s))
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    File "/usr/lib/pymodules/python2.6/shapely/ops.py", line 38, in polygonize
      for g in collection.geoms:
    File "/usr/lib/pymodules/python2.6/shapely/geometry/base.py", line 53, in wrapper
      raise ValueError("Null/empty geometry supports no operations")
  ValueError: Null/empty geometry supports no operations