array_ndims never returns zero

Hi,
Reading
https://stackoverflow.com/questions/48022753/why-does-array-ndimsarray-produce-null#48022980
confused me much - why array_ndims never returns zero indeed?..
select char_length('') returns zero and according to
https://www.postgresql.org/docs/current/static/functions-string.html it
shows the "Number of characters in string ",on the other hand
https://www.postgresql.org/docs/10/static/functions-array.html
array_ndims "returns
the number of dimensions of the array",but
https://github.com/postgres/postgres/blob/master/src/backend/utils/adt/arrayfuncs.c#L1635

if (AARR_NDIM(v) <= 0 || AARR_NDIM(v) > MAXDIM)
PG_RETURN_NULL(); so the question - do you think it makes sense?.. What is
the logic behind it?
Please advise where to address it if I chose the wrong channel, as It's
probably not a bug, but a feature?..

Regards