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 IColumn * col_then_untyped = block.getByPosition(arguments[1]).column.get();
        const IColumn * col_else_untyped = block.getByPosition(arguments[2]).column.get();
        const ColumnString * col_then = checkAndGetColumn<ColumnString>(col_then_untyped);
        const ColumnString * col_else = checkAndGetColumn<ColumnString>(col_else_untyped);
        const ColumnFixedString * col_then_fixed = checkAndGetColumn<ColumnFixedString>(col_then_untyped);
        const ColumnFixedString * col_else_fixed = checkAndGetColumn<ColumnFixedString>(col_else_untyped);
        const ColumnConst * col_then_const = checkAndGetColumnConst<ColumnString>(col_then_untyped);
        const ColumnConst * col_else_const = checkAndGetColumnConst<ColumnString>(col_else_untyped);
        const ColumnConst * col_then_const_fixed = checkAndGetColumnConst<ColumnFixedString>(col_then_untyped);
        const ColumnConst * col_else_const_fixed = checkAndGetColumnConst<ColumnFixedString>(col_else_untyped);
        const PaddedPODArray<UInt8> & cond_data = cond_col->getData();
        size_t rows = cond_data.size();
        if (isFixedString(block.getByPosition(result).type))
        {
            
            auto col_res_untyped = block.getByPosition(result).type->createColumn();
            ColumnFixedString * col_res = assert_cast<ColumnFixedString *>(col_res_untyped.get());
            auto sink = FixedStringSink(*col_res, rows);