Fix for checking rules

`$item->ruleName === null` don't works with `DbManager`.

Generates `yii\base\InvalidConfigException` with message `Rule not found:`, although item is empty

```
yii\rbac\Role#1
(
    [type] => '0'
    [name] => 'projects.index'
    [description] => 'Проекты: список'
    [ruleName] => null
    [data] => null
    [createdAt] => '0'
    [updatedAt] => '0'
)
```