Updating a Nullable column value by using alter .... where ... causes to set another value!

ClickHouse version: 20.6.3.28
Engine: MergeTree
OS: Ubuntu 18.04.3 LTS

Steps to reproduce:
```sql
CREATE TABLE table1(
	id int,
	price Nullable(Int32)
)
ENGINE = MergeTree()
PARTITION BY id
ORDER BY (id);
INSERT INTO table1 (id, price) VALUES (1, 100);
ALTER TABLE table1  update price = 150 where id=1;
SELECT * FROM table1;
```
We expect the price to be `150`, but it's `93148480`:

```
┌─id─┬────price─┐
│  1 │ 93148480 │
└────┴──────────┘
```
Using `Decimal(9,2)` field instead of `Int32` would set the value  `932342.88` always. It seems there's a fixed value for each data type.

If we don't use such a `where` clause, it works correctly:
```sql
ALTER TABLE db1.table1  update price = 150 where 1=1;
┌─id─┬─price─┐
│  1 │   150 │
└────┴───────┘
```
const ColumnUInt8 * cond_col, Block & block, const ColumnNumbers & arguments, size_t result, size_t input_rows_count)
    {
        
        const ColumnWithTypeAndName & arg1 = block.getByPosition(arguments[1]);
        const ColumnWithTypeAndName & arg2 = block.getByPosition(arguments[2]);
        DataTypePtr common_type = getLeastSupertype({arg1.type, arg2.type});
        ColumnPtr col_then = castColumn(arg1, common_type);
        ColumnPtr col_else = castColumn(arg2, common_type);
        MutableColumnPtr result_column = common_type->createColumn();
        result_column->reserve(input_rows_count);
        bool then_is_const = isColumnConst(*col_then);
        bool else_is_const = isColumnConst(*col_else);
        const auto & cond_array = cond_col->getData();
        if (then_is_const && else_is_const)
        {
            const IColumn & then_nested_column = assert_cast<const ColumnConst &>(*col_then).getDataColumn();
            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(then_nested_column, 0);
                else
                    result_column->insertFrom(else_nested_column, 0);
            }
        }