Skip is_scalar check for null values when imploding a multiline address attribute.

Author: Zaahed

app/code/Magento/Customer/Model/Address/AbstractAddress.php

@@ -331,7 +331,7 @@ protected function _implodeArrayValues($value)
 
             $isScalar = true;
             foreach ($value as $val) {
-                if (!is_scalar($val)) {
+                if ($val !== null && !is_scalar($val)) {
                     $isScalar = false;
                     break;
                 }