Indeed, it does call triangular(0), which returns 0, which is added to the recursive summation, causing no change to the value:
triangular(4) =
triangular(3) + 4 =
triangular(2) + 3 + 4 =
triangular(1) + 2 + 3 + 4 =
triangular(0) + 1 + 2 + 3 + 4 =
0 + 1 + 2 + 3 + 4 =
10

i ended up solving it this way, but i don't understand why this doesn't return 0. if we are recursively calling n-1 wont the function continue until n < 1?

loved this kata! spent about 20 minutes towards the end realizing that i needed to convert to float for the last few tests. haha.