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)
```



`is_null()` is slower than a strict literal equality check with `null`. If you want a looser "falsy" check, then `empty()` is a better option.