string_to_array with an empty input string

Looking through Pavel's string_to_array patch, I notice that the new
version of string_to_array returns an empty (zero-element) array when
the input string is of zero length, whereas the traditional version
returned NULL instead.  The patch fails to emulate the old behavior
exactly, but rather than fix it, I wonder if we shouldn't make the two
versions behave the same.  (If we don't have them doing the same thing,
we're going to have to document the difference...)  The NULL result
seems a bit peculiar to me, and the empty-array result saner.  Given
the general inconsistency surrounding empty arrays elsewhere, I think
this wouldn't be a very problematic change for most users.

Thoughts?
        regards, tom lane