Add missing attributes initialization to avoid throwing foreach error

Author: wojtekn

app/code/Magento/Eav/Model/Form.php

@@ -323,6 +323,8 @@ public function getAttributes()
         if ($this->_attributes === null) {
             $this->_attributes = [];
             $this->_userAttributes = [];
+            $this->_systemAttributes = [];
+            $this->_allowedAttributes = [];
             /** @var $attribute \Magento\Eav\Model\Attribute */
             foreach ($this->_getFilteredFormAttributeCollection() as $attribute) {
                 $this->_attributes[$attribute->getAttributeCode()] = $attribute;