test for finding by null

Fix for the following issue:

```PHP
$author = Author::find(0);
// throws RecordNotFound exception (because count(0) == 1)

$author = Author::find(null);
// does not throw an exception (because count(null) == 0)
```



Almost there, there is no need to convert values to an array if we're going to join it directly after. Or am I missing something?

```php
if (is_array($values))
	$values = join(',', $values);
```