I'm using beta 22 and have capitalization = original in theme_settings{} and inventory_list_empty = "nothing" in system_messages{}. When I take an inventory at the start of the game (when the player is empty-handed), the list of carried items is displayed as "Nothing". The capitalisation setting is being ignored and the original string is being converted to sentence case. (This was confirmed by using other strings for test purposes.)
Incidentally, I have a lead-in phrase, so "nothing" is used for consistency with all the objects, which are also lower case.