public static Sorting fromString(String sortSpec) {
        if (sortSpec == null) return null;
        if ("".equals(sortSpec)) return null;
        return new Sorting(sortSpec);
    }
Creates a new sorting from the given string and returns it, or returns null if the argument does not contain
any sorting criteria (e.g it is null or the empty string)
container-search/src/main/java/com/yahoo/search/query/Sorting.java