IsNullable does not retain null when we return nil in expressions

[theodesp]

I'm trying to implement nullif and I assign the return of IsNullable as true:

func (ni *NullIf) IsNullable() bool { return true }

// Type implements the sql.Expression interface.
func (ni *NullIf) Type() sql.Type {
	return ni.Left.Type()
}

Then in the code at some point in the Eval function I return nil to indicate that the 2 values are equal:

        expr:= expression.NewEquals(ni.Left, ni.Right)
	areEqual, err := expr.Eval(ctx, row)
	if err != nil {
		return nil, err
	}

	if areEqual.(bool) {
		return nil, nil
	}
	return left, nil

However when I use numbers then the nil become 0 which is wrong

+--------------+
| nullif(1, 1) |
+--------------+
|            0 |
+--------------+