Changed representation of uncategorized to ""

Ideally I'd like None to represent no category but empty string fits
better with what is on the form for the individual dropdowns. So
let's make that consistent.

django/website/hid/tabs/view_and_edit_table.py

@@ -257,7 +257,7 @@ def _handle_batch_action(request, batch_action, selected):
     if batch_action == REMOVE_QTYPE_COMMAND:
         _categorize_items(request,
                           selected,
-                          None)
+                          '')
         return
 
     messages.error(request, _("Unknown batch action '%s'" % batch_action))
@@ -343,7 +343,7 @@ def _add_items_categories(request, items):
     failed = 0
     for item_id, taxonomy_slug, term_name in items:
         try:
-            if term_name:  # TODO not None
+            if term_name:
                 transport.items.add_term(
                     item_id,
                     taxonomy_slug,