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)
```



What is wrong with `is_null`? 

Because: 
```php
is_null(null) # => true
is_null(false) # => false
is_null(0) # => false
```