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 │
└────┴───────┘
```
{
        size_t size = cond.size();
        auto col_res = ColVecResult::create(size);
        typename ColVecResult::Container & res = col_res->getData();
        for (size_t i = 0; i < size; ++i)
            res[i] = cond[i] ? bigint_cast<ResultType>(a[i]) : bigint_cast<ResultType>(b[i]);
        block.getByPosition(result).column = std::move(col_res);
    }
    static void vectorConstant(const ArrayCond & cond, const ArrayA & a, B b, Block & block, size_t result, UInt32)
    {
        size_t size = cond.size();
        auto col_res = ColVecResult::create(size);
        typename ColVecResult::Container & res = col_res->getData();
        for (size_t i = 0; i < size; ++i)
            res[i] = cond[i] ? bigint_cast<ResultType>(a[i]) : bigint_cast<ResultType>(b);
        block.getByPosition(result).column = std::move(col_res);
    }
    static void constantVector(const ArrayCond & cond, A a, const ArrayB & b, Block & block, size_t result, UInt32)