Fix #970: Correct validation of TABLIST_HEADERS dialogs

[moreveal]

trafficstars

Forcibly sends the first item selection (listitem = 0) to dialog if there are no items at all (since in this case the client sends the listitem of the last dialog sent). In addition, an item will not be displayed on the client if it is empty (\n one after the other) - this case is now also taken into account. lines now exactly reflects its name - it is the number of items, not the number of line breaks.