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)
```



Oh, and I probably moved it because line 1678 requires an array, but the operation to guarantee an array was happening inside a logical check. It may be that the move wasn't needed, but I found it more clear to read.