@Override
        public String getValue(Map<String, String> context, Properties substitution) {
            Object value = substitution.get(propertyName, context, substitution);
            if (value == null) return "";
            return String.valueOf(value);
        }
Returns this string in original (unsubstituted) form
container-search/src/main/java/com/yahoo/search/query/profile/SubstituteString.java