array_ndims never returns zero

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