code: public static FieldFilter create(String fieldParam) {
      if (fieldParam == null || fieldParam.isEmpty()) {
        return new FieldFilter(Sets.<String>newHashSet());
      }
      return new FieldFilter(Sets.newHashSet(fieldParam.split(",")));
    }
docs: Creates a new {@code FieldFilter} to handle empty or null params.
  @param fieldParam comma separated list of fields to keep when executing the filter; should be
  in the format"id,inner_model/id,other_thing" corresponding to field names in the JSON
  @return a new FieldFilter to be applied to JSON