```
select tupleElement(CAST(tuple(1,2),'Tuple(x UInt64, y UInt64)'), 'x') 
-- ok

select tupleElement(CAST(tuple(1,2),'Tuple(x UInt64, y UInt64)'), 'z')

-- fails
-- Code: 10. DB::Exception: Received from localhost:9000. DB::Exception: Tuple doesn't have element with name 'z': While processing tupleElement(CAST((1, 2), 'Tuple(x UInt64, y UInt64)'), 'z'). (NOT_FOUND_COLUMN_IN_BLOCK)
```
Proposal - add the 3rd parameter to the tupleElement function and return it if tuple doesn't have a member.

Why? Tuple comes from object('JSON'), sometimes some fields are missing in some jsons.
#include <Functions/IFunction.h>
#include <Functions/FunctionFactory.h>
#include <Functions/FunctionHelpers.h>
#include <DataTypes/IDataType.h>
#include <DataTypes/DataTypeTuple.h>
#include <DataTypes/DataTypeArray.h>
#include <Columns/ColumnTuple.h>
#include <Columns/ColumnArray.h>
#include <Columns/ColumnString.h>
#include <Columns/ColumnsNumber.h>
#include <Common/assert_cast.h>
#include <memory>
namespace DB
{
namespace ErrorCodes
{
    extern const int ILLEGAL_TYPE_OF_ARGUMENT;
    extern const int ILLEGAL_INDEX;
}
namespace
{
/** Extract element of tuple by constant index or name. The operation is essentially free.
  * Also the function looks through Arrays: you can get Array of tuple elements from Array of Tuples.
  */
class FunctionTupleElement : public IFunction