However, when all of one person type have been taken and a player still has a card for that person remaining, that player should still be able to play that person card and take nothing. As far as I can tell, you are only able to do so on BGA if it is your last card remaining (you have no persons you can recruit that would allow you to take a tile). However, the rules of the game do not prohibit you from playing a card that would give you nothing even if you have other cards to play. They only say that each player "plays 1 person card from their hand onto the common discard pile on the board. The player then takes the matching person tile from the board and places it below any of their palaces"... "If a player plays a card showing a courtier whose tile is no longer available on the board, then that player gets nothing (i.e. does not get a substitute person)."
At one time I think I remember seeing an outline in the person area for empty person stacks with a "x0" indicator (though I'm not sure if it was intentional) but that is no longer the case.
Thanks in advance!