parsec: string_list: fix handling of null value

A null value supplied to a `string_list` setting was not returning a
list of string, but a null string. This fixes the problem.
Well, I had assumed we wouldn't be testing ```value[0]``` on an empty string - a small if-block at the top of the function would prevent that ... but given that you didn't code it that way, then fair enough :grin: