@Nullable
    private static List<String> singletonOrNull(final String item) {
        return StringUtils.isNotBlank(item) ? Collections.singletonList(item) : null;
    }
Get user rating for a given guid or geocode. For a guid first the ratings cache is checked
before a request to gcvote.com is made.
main/src/main/java/cgeo/geocaching/gcvote/GCVote.java