Product Name is null after left join with custom table via Event Observer

Question

I am making use of Marketplace extension and wanted to show seller name after product name in Category list page. I have used "catalog_block_product_list_collection" events

<event name="catalog_block_product_list_collection">
    <observer name="add_seller_info_product_list_collection" instance="Vendor\Page\Observer\AddSellerInfoListCollection" />
</event>

And Observer Class

namespace Vendor\Page\Observer;

class AddSellerInfoListCollection 
implements \Magento\Framework\Event\ObserverInterface
{
    protected $_objectManager;
    public function __construct(
        \Magento\Framework\App\Helper\Context $context,
        \Magento\Framework\ObjectManagerInterface $objectManager){
        $this->_objectManager = $objectManager;
    }

    public function execute(\Magento\Framework\Event\Observer $observer){
        $collection = $observer->getCollection();
        $seller = $this->_objectManager->create(
            'Magento\Catalog\Model\ResourceModel\Product\Collection'
        )->getTable('customer_grid_flat');

        $sellerProduct = $this->_objectManager->create(
            'Magento\Catalog\Model\ResourceModel\Product\Collection'
        )->getTable('marketplace_product');

        $sellerProfile = $this->_objectManager->create(
            'Magento\Catalog\Model\ResourceModel\Product\Collection'
        )->getTable('marketplace_userdata');


        $collection->getSelect()->joinLeft(
                $sellerProduct.' as seller_product',
                'e.entity_id = seller_product.mageproduct_id'
            );

        $collection->getSelect()->joinLeft(
                $seller.' as seller',
                'seller_product.seller_id = seller.entity_id',
                array('seller_name'=>'seller.name')
            );

        $collection->getSelect()->joinLeft(
                $sellerProfile.' as sellerprofile',
                'sellerprofile.seller_id = seller.entity_id',
                array('seller_shop_url'=>'sellerprofile.shop_url')
        );       
        return $this;
    }    
}

I get proper results when a product is associated with some seller, but if there is a product not associated with any seller then all product attributes ( like name, url, etc) are not loaded.