Magento 2: Display guest customer name on custom admin grid

This is the dataprovider code which gets the customer’s info in my admin grid. It doesnt show the customer name for guest customer. How can I show customer name for guests customers?

    /**
     * @return array
     */
    public function getData()
    {
        $collection = $this->getCollection();
        $filterMap = $this->getFilterMap();
        $storeIds = $this->dataHelper->getStoreIds();

        $collection->addOrderData()
            ->addCustomerData()
            ->addAdminData()
            ->addLastReplyData();

        if (false === $this->isArchive()) {
            $collection->addNotArchiveFilter();
        } else {
            $collection->addArchiveFilter();
        }

        foreach ($filterMap as $filter => $alias) {
            $collection->addFilterToMap($filter, $alias);
        }

        if (!empty($storeIds)) {
            $collection->addFieldToFilter('o.store_id', ['in' => $storeIds]);
        }

        return $collection->toArray();
    }

    /**
     * @return bool|false|int
     */
    public function isArchive()
    {
        return mb_strpos($this->request->getServerValue('HTTP_REFERER'), 'returnsarchive');
    }

    /**
     * @param MagentoFrameworkApiFilter $filter
     * @return $this
     */
    public function addFilter(MagentoFrameworkApiFilter $filter)
    {
        if ('fulltext' === $filter->getConditionType()) {
            $this->fulltextFilter->apply($this->getCollection(), $filter);
        } else {
            $filterMap = $this->getFilterMap();

            if (isset($filterMap[$filter->getField()])) {
                $filter->setField($filterMap[$filter->getField()]);
            }

            parent::addFilter($filter);
        }

        $this->searchCriteriaBuilder->addFilter($filter);

        return $this;
    }

    /**
     * @return array
     */
    private function getFilterMap()
    {
        return $filterMap = [
            'increment_id' => 'main_table.increment_id',
            'created_at'   => 'main_table.created_at',
            'order_increment_id' => 'o.increment_id',
            'order_date' => 'o.updated_at',
            'entity_id' => 'main_table.entity_id',
            'customer_name' => 'c.name',
            'reply_at' => 'rm.created_at',
            'status' => 'main_table.status',
            'store_id' => 'o.store_id',
            'manager_name' => new Zend_Db_Expr('CONCAT(au.`firstname`, " ", au.`lastname`)')
        ];
    }

    /**
     * @return $this
     */
    public function getSearchCriteria()
    {
        if (!$this->searchCriteria) {
            $this->searchCriteria = $this->searchCriteriaBuilder->create();
            $this->searchCriteria->setRequestName($this->name);
        }
        return $this->searchCriteria;
    }

    /**
     * Returns Search result
     *
     * @return MagentoFrameworkApiSearchSearchResultInterface
     */
    public function getSearchResult()
    {
        return $this->reportingInterface->search($this->getSearchCriteria());
    }
}