Length returns NULL ?

There are some functions called ...length, but only array_length returns
NULL on empty array, why ?

select array_length('{}'::text[],1), -->NULL
jsonb_array_length('[]'), -->0
bit_length(''), -->0
octet_length(''), -->0
length(''), -->0
char_length(''), -->0
length(B''); -->0

I know, it is documented, but the question is, why does it work differently
?

array_length ( anyarray, integer ) → integer
Returns the length of the requested array dimension. (Produces NULL instead
of 0 for empty or missing array dimensions.)

array_length(array[1,2,3], 1) → 3
array_length(array[]::int[], 1) → NULL
array_length(array['text'], 2) → NULL