Update where.php to improve WHERE IN query

Update where.php to improve WHERE IN query experience:

When
```
DB::select()->from('users')->where('id', 'IN', array(1,2,3));
```
Kohana Database will act as
```
SELECT * FROM `users` WHERE `id` IN (1,2,3)
```

But when
```
DB::select()->from('users')->where('id', 'IN', array());
```
The SQL:
```
SELECT * FROM `users` WHERE `id` IN ()
```
will run a fault.

So the change is
When
```
DB::select()->from('users')->where('id', 'IN', array());
```
The SQL will be:
```
SELECT * FROM `users` WHERE 1 = 0
```