dont crash CustomerSetupUpgrade on users without password

there may be modules or setups, which work with users not having a password at all
(cherry picked from commit 539b80f)

Author: Flyingmana

app/code/Magento/Customer/Setup/UpgradeData.php

@@ -333,6 +333,9 @@ private function upgradeHash($setup)
 
         $customers = $setup->getConnection()->fetchAll($select);
         foreach ($customers as $customer) {
+            if ($customer['password_hash'] === null) {
+                continue;
+            }
             list($hash, $salt) = explode(Encryptor::DELIMITER, $customer['password_hash']);
 
             $newHash = $customer['password_hash'];