Magento do not merge Customer Quote, if current quote is not empty

Observer.php

<?php
class Observer {
    public function load_customer_quote_before($observer)
    {
        $checkout_session = $observer->getEvent()->getCheckoutSession();
        if($checkout_session->getQuote()->getItemsCount() < 1) // check if current quote has items
        {
            return $this;
        }
        
        $customerQuote = Mage::getModel('sales/quote')
            ->setStoreId(Mage::app()->getStore()->getId())
            ->loadByCustomer(Mage::getSingleton('customer/session')->getCustomerId());
        $customerQuote->delete();
        return $this;
    }
}

Event in File: app/code/core/Mage/Checkout/Model/Session.php
in Method: loadCustomerQuote

Mage::dispatchEvent('load_customer_quote_before', array('checkout_session' => $this));