Update categories/templatetags/category_tags.py

Added str() to line 49 to fix an error where .strip("'\"") in get_category is getting called on a non-string category_string.
Update categories/templatetags/category_tags.py

Added NoneType check to display_drilldown_as_ul on line 188 to fix NoneType error.
Merge pull request #64 from iepathos/master

Fixed NoneType error in templatetags