Reproduction steps:
```
CREATE TABLE table_left                                                                                                                                                                                              
(                                                                                                                                                                                                                    
    `date` Date,                                                                                                                                                                                                     
    `uid` String                                                                                                                                                                                                     
)                                                                                                                                                                                                                    
ENGINE = MergeTree()                                                                                                                                                                                                 
ORDER BY (date, uid)          
                                                                                                                                                                                       
CREATE TABLE table_right                                                                                                                                                                                             
(                                                                                                                                                                                                                    
    `date` Date,                                                                                                                                                                                                     
    `value` Decimal(18, 10)                                                                                                                                                                                          
)                                                                                                                                                                                                                    
ENGINE = MergeTree()                                                                                                                                                                                                 
ORDER BY date

INSERT INTO table_left values(toDate('2019-10-01'), 'hello');
INSERT INTO table_right values (toDate('2019-10-01'), 1), (toDate('2029-10-01'), 2);

SET join_use_nulls = 1;

SELECT ifNull(subquery_right.alias_value, 0)
FROM table_left
FULL OUTER JOIN 
(
    SELECT
        date,
        value AS alias_value
    FROM table_right
) AS subquery_right
ON table_left.date = subquery_right.date

Received exception from server (version 21.3.1):
Code: 43. DB::Exception: Received from localhost:9000. DB::Exception: Incompatible types of arguments corresponding to two conditional branches: while executing 'FUNCTION ifNull(alias_value :: 0, 0 :: 1) -> ifNull(alias_value, 0) Decimal(18, 10) : 2'. 
```
else if (then_is_const)
        {
            const IColumn & then_nested_column = assert_cast<const ColumnConst &>(*col_then).getDataColumn();
            for (size_t i = 0; i < input_rows_count; ++i)
            {
                if (cond_array[i])
                    result_column->insertFrom(then_nested_column, 0);
                else
                    result_column->insertFrom(*col_else, i);
            }
        }
        else if (else_is_const)
        {
            const IColumn & else_nested_column = assert_cast<const ColumnConst &>(*col_else).getDataColumn();
            for (size_t i = 0; i < input_rows_count; ++i)
            {
                if (cond_array[i])
                    result_column->insertFrom(*col_then, i);
                else
                    result_column->insertFrom(else_nested_column, 0);
            }
        }
        else
        {
            for (size_t i = 0; i < input_rows_count; ++i)
                result_column->insertFrom(cond_array[i] ? *col_then : *col_else, i);
        }
        return result_column;
    }
    ColumnPtr executeForConstAndNullableCondition(const ColumnsWithTypeAndName & arguments, const DataTypePtr & result_type, size_t ) const