public HCatTable nullDefinedAs(char nullChar) {
    return serdeParam(serdeConstants.SERIALIZATION_NULL_FORMAT, Character.toString(nullChar));
  }
See <i>row_format</i> element of CREATE_TABLE DDL for Hive.
hcatalog/webhcat/java-client/src/main/java/org/apache/hive/hcatalog/api/HCatTable.java