very odd choice and to me very counterintuitive. When the primary rules is "each cell indicates how many different numbers its arrow(s) points to" to me a cell with no arrows thus points to zero numbers.
The less rules a puzzle needs and the simpeler they are, the more beautiful a genre is. Adding an exepticion to such rule uglifies the genre. And it doesn't even make the puzzles more challenging. Because when a cell doesn't point to anything, you know for sure it can be the last cell to fill in. You can ignore it completely while solving the puzzle.