void appendMissingValue(StringBuilder b)
        {
            if (missingValue == null) {
                b.append("\"\"");
            } else {
                if (needQuotes(missingValue)) {
                    appendQuoted(b, missingValue);
                } else {
                    b.append(missingValue);
                }
            }
        }
Returns the serial form of this which contains all information needed to reconstruct this sorter
container-search/src/main/java/com/yahoo/search/query/Sorting.java