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)
```



Hmm, I don't think there really is too much I can do without making the code more complex; I can't do all the error checking up front, because we have to do some work before we can know there is an error. I switched it back to using the separate call to get_called_class. Can we live with it?