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'. 
```
if (col_then_fixed && col_else_fixed)
                conditional(FixedStringSource(*col_then_fixed), FixedStringSource(*col_else_fixed), sink, cond_data);
            else if (col_then_fixed && col_else_const_fixed)
                conditional(FixedStringSource(*col_then_fixed), ConstSource<FixedStringSource>(*col_else_const_fixed), sink, cond_data);
            else if (col_then_const_fixed && col_else_fixed)
                conditional(ConstSource<FixedStringSource>(*col_then_const_fixed), FixedStringSource(*col_else_fixed), sink, cond_data);
            else if (col_then_const_fixed && col_else_const_fixed)
                conditional(ConstSource<FixedStringSource>(*col_then_const_fixed),
                            ConstSource<FixedStringSource>(*col_else_const_fixed), sink, cond_data);
            else
                return nullptr;
            return col_res_untyped;
        }
        if (isString(result_type))
        {
            
            auto col_res = ColumnString::create();
            auto sink = StringSink(*col_res, rows);
            if (col_then && col_else)
                conditional(StringSource(*col_then), StringSource(*col_else), sink, cond_data);
            else if (col_then && col_else_const)
                conditional(StringSource(*col_then), ConstSource<StringSource>(*col_else_const), sink, cond_data);
            else if (col_then_const && col_else)
                conditional(ConstSource<StringSource>(*col_then_const), StringSource(*col_else), sink, cond_data);
            else if (col_then_const && col_else_const)
                conditional(ConstSource<StringSource>(*col_then_const), ConstSource<StringSource>(*col_else_const), sink, cond_data);
            else if (col_then && col_else_fixed)
                conditional(StringSource(*col_then), FixedStringSource(*col_else_fixed), sink, cond_data);
            else if (col_then_fixed && col_else)
                conditional(FixedStringSource(*col_then_fixed), StringSource(*col_else), sink, cond_data);