Support null value for custom attributes.

Author: meng-tian

lib/internal/Magento/Framework/Webapi/ServiceInputProcessor.php

@@ -260,7 +260,7 @@ private function processCustomAttribute($customAttribute)
             throw new SerializationException(new Phrase('There is an empty custom attribute specified.'));
         } elseif (!$customAttributeCode) {
             throw new SerializationException(new Phrase('A custom attribute is specified without an attribute code.'));
-        } elseif (!isset($customAttribute[AttributeValue::VALUE])) {
+        } elseif (!array_key_exists(AttributeValue::VALUE, $customAttribute)) {
             throw new SerializationException(
                 new Phrase('Value is not set for attribute code "' . $customAttributeCode . '"')
             );