Tharos/LeanMapper — pull request #5 by besanek (2013-07-14 18:57:52+00:00)
Title: Removed empty queries with nullable relationship.

Situation: An entity with nullable one reference. If you find only one entity that has this property is set to null, it generates the empty query.

Example
-------------
```php
$book = new Book;
$book->author = NULL;
$book->title = "The book written by anonym";
$bookRepository->persist($book);

///

$bookRepository->find($book->id);
```

Generates :
```sql
SELECT `file`.*
FROM `file`
WHERE `file`.`id` IN (NULL)
```