[1.0.X] Return None for null date/time fields in OGR-supported data sources. ...

[1.0.X] Return None for null date/time fields in OGR-supported data sources.  Thanks to Ariel Mauricio Nunez Gomez for bug report and initial patch.

Backport of r9711 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9712 bcc190cf-cafb-0310-a4f2-bffc1f526a37