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

When
```php
    /**
     * Initialize method for model.
     */
    public function initialize()
    {
        $this->setSchema("");
    }
```
```php
$result= $this->getDI()->get('modelsManager')
    ->createBuilder()
    ->columns('*')
    ->from('Accounts')
    ->join('Contacts', 'Contacts.account_id = Accounts.id')
   ->limit(10)
   -> getQuery()->execute();
```
SQL:
```SQL
SELECT "accounts"."id" AS "_accounts_id", "accounts"."phone" AS "_accounts_phone", "contacts"."id" AS "_contacts_id", "contacts"."account_id" AS "_contacts_account_id" FROM "accounts" INNER JOIN ""."contacts" ON "contacts"."account_id" = "accounts"."id"  LIMIT 10
```