[CH] var_samp returns NaN instead of NULL when effective row count < 2

### Backend

CH (ClickHouse)

### Bug description

select var_samp(id) from range(1);
     select covar_samp(id, id) from range(1);

    Expected NULL

    Actual NaN

### Gluten version

_No response_

### Spark version

None

### Spark configurations

_No response_

### System information

_No response_

### Relevant logs

```bash

```