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)
```



If we move the `$values = join(',',$values);` from below to here, we can simply use `"Couldn't find $class with ID=$values"` for the exception message