ENGCOM-3332: Added $_customerCollection property to Storage class #144

 - Merge Pull Request magento-engcom/import-export-improvements#144 from maxalmonte14/import-export-improvements:improvement/supress_undefined_customerCollection_property_error
 - Merged commits:
   1. 5181674
   2. b272c4e
   3. e973095
   4. 2d45eb3
   5. bb946c4

Author: magento-engcom-team

app/code/Magento/CustomerImportExport/Model/ResourceModel/Import/Customer/Storage.php

@@ -13,6 +13,9 @@
 use Magento\ImportExport\Model\ResourceModel\CollectionByPagesIteratorFactory;
 use Magento\ImportExport\Model\ResourceModel\CollectionByPagesIterator;
 
+/**
+ * Storage to check existing customers.
+ */
 class Storage
 {
     /**
@@ -48,6 +51,11 @@ class Storage
      */
     private $customerCollectionFactory;
 
+    /**
+     * @var CustomerCollection
+     */
+    public $_customerCollection;
+
     /**
      * @param CustomerCollectionFactory $collectionFactory
      * @param CollectionByPagesIteratorFactory $colIteratorFactory
@@ -112,6 +120,8 @@ private function loadCustomersData(array $customerIdentifiers)
     }
 
     /**
+     * Add a customer by an array
+     *
      * @param array $customer
      * @return $this
      */