Fix bug where empty string returns nothing

[johnchase]

If the existing list passed to create cualids contains an empty string the function returns nothing:

for id_ in cualid.create_ids(n=7, id_length=4, existing_ids=['']):
    print(e)

If an empty string is valid it should not return nothing, if it is not valid it should throw a warning. I don't feel an empty string should be considered valid