public RelDataType getReturnType(RelDataTypeFactory typeFactory) {
    return typeFactory.createTypeWithNullability(
        typeFactory.createSqlType(SqlTypeName.ANY), true);
  }
<code>Sum0</code> is an aggregator which returns the sum of the values which
go into it like <code>Sum</code>. It differs in that when no non null values
are applied zero is returned instead of null. Can be used along with <code>
Count</code> to implement <code>Sum</code>.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/functions/HiveSqlSumEmptyIsZeroAggFunction.java