 WHERE expr IN (...) Broken for empty list

Worked in: 2.0.0rc8
Broken in: 2.0.0, 2.0.1

``` javascript
// Note that list is empty
var list = [];
model.findAll(... {where: {col: list}})...
```

produces in 2.0.0rc8 with MySQL:

``` SQL
`Model`.`col` IN (NULL)
```

produces in 2.0.0-2.0.1 with MySQL:

``` SQL
`Model`.`col` IN ()
```

Which breaks the MySQL-syntax.
