[#2845] Allow passing empty lists to group_update

If, for example, a group dict with a 'packages' key with an empty list
as the value is passed to group_update, then set the group's packages to
an empty list (i.e. remove any packages from the group), even if
'allow_partial_update': True was specified in the context (previously
such an empty list would be ignored and the group's packages would not
change). Also applies to other group fields e.g. tags, users, groups, etc.

ckan/lib/dictization/model_save.py

@@ -319,10 +319,13 @@ def group_member_save(context, group_dict, member_table_name):
     model = context["model"]
     session = context["session"]
     group = context['group']
-    entity_list = group_dict.get(member_table_name, [])
+    entity_list = group_dict.get(member_table_name, None)
 
-    if entity_list == [] and context.get('allow_partial_update', False):
-        return {'added': [], 'removed': []}
+    if entity_list is None:
+        if context.get('allow_partial_update', False):
+            return {'added': [], 'removed': []}
+        else:
+            entity_list = []
 
     entities = {}
     Member = model.Member