For documents that do not have a value set for a given field, most field types
return None (or [] in the case of ListField). This makes it easy to test
whether a field has been set using "if doc.field". FileFields, on the other
hand, always return a GridFSProxy. Adding GridFSProxy.__nonzero__ which simply
checks for a grid_id allows the same boolean-test pattern for FileFields, as
well.
