> Try this:
>
> #+CAPTION: Attendances for April
> |---------+-------------------+---+---+---+---+----+-------|
> | Account | Name              | 1 | 3 | 5 | 8 | 10 | Total |
> |---------+-------------------+---+---+---+---+----+-------|
> |    1234 | Cárdenas, Lázaro  | X |   | X | X |    |     3 |
> |    5678 | Madero, Francisco | X | X | X | X |    |     4 |
> |    1544 | Villa, Pancho     |   |   |   |   |    |     0 |
> |    0113 | Zapata, Emiliano  |   | X | X |   |    |     2 |
> |---------+-------------------+---+---+---+---+----+-------|
> #+tblfm: @II+1$>..@III-1$>='(length (delq "" '($<<<..$>>)))

Interesting, my org-mode version behaves differently, and still gives
'1' for the empty row with your version:

Substitution history of formula
(...)
@r$c-> '(length (delq "" '("0")))
$1->   '(length (delq "" '("0")))
Result: 1