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)
```



Well, this isn't my code, and I'm not aware of all the possible cases, but this seems to catch a wide variety of them. Imagine the case where an array with one item in it is passed in. Then count($values) == 1, the force to array doesn't happen, but join still works. If a single scalar value is passed in, count($values) is still 1, it gets forced to array, and the join still works.