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)
```



Sure, but then that's an extra function call for non-error cases. Given that we're down to debating is_null vs $var === null, I figured this would be preferable.