Fix test

`base_amount` is always returned as null
https://github.com/magento/graphql-ce/blob/4013a390348bf15ae48ddcaeecadae402351cf33/app/code/Magento/QuoteGraphQl/Model/Resolver/ShippingAddress/AvailableShippingMethods.php#L100

Author: pmclain

dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/Guest/GetAvailableShippingMethodsTest.php

@@ -67,6 +67,7 @@ public function testGetAvailableShippingMethods()
                 'value' => 10,
                 'currency' => 'USD',
             ],
+            'base_amount' => null,
         ];
         self::assertEquals(
             $expectedAddressData,