Remove unexpanded lists when extending lists

Instead of overwriting the list value with cf_null, which is then
no longer stripped out during later processing, take unexpanded
entries out of the list.

Extend existing acceptance test.

Fixes redmine 6541
Add another situation to list expansion test.
Merge pull request #1930 from vohi/redmine/6541

Redmine 6541 - Remove unexpanded lists when extending lists