Fix BUG Model\Query::_getJoins when set schema empty string

When
```php
class Contacts extends \Phalcon\Mvc\Model
{
    /**
     * Initialize method for model.
     */
    public function initialize()
    {
        $this->setSchema("");
    }
}
// ...
$result= $this->getDI()->get('modelsManager')
    ->createBuilder()
    ->columns('*')
    ->from('Accounts')
    ->join('Contacts', 'Contacts.account_id = Accounts.id')
   ->limit(10)
   -> getQuery()->execute();
```
SQL error in `""."contacts"`:
```SQL
SELECT ... FROM "accounts" INNER JOIN ""."contacts" ON "contacts"."account_id" = "accounts"."id"  LIMIT 10
```