Prevent undefined index warning before setting shipping method

`list` would cause and undefined index error when address rates had been
collected and the shipping method was not set. This commit checks that the
shipping method is set prior to resolving.

Fixes #657

Author: pmclain

app/code/Magento/QuoteGraphQl/Model/Resolver/ShippingAddress/SelectedShippingMethod.php

@@ -49,7 +49,7 @@ public function resolve(Field $field, $context, ResolveInfo $info, array $value
         $carrierTitle = null;
         $methodTitle = null;
 
-        if (count($rates) > 0) {
+        if (count($rates) > 0 && !empty($address->getShippingMethod())) {
             list($carrierCode, $methodCode) = explode('_', $address->getShippingMethod(), 2);
 
             /** @var Rate $rate */

dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/Customer/GetSelectedShippingMethodTest.php

@@ -88,6 +88,40 @@ public function testGetSelectedShippingMethod()
         self::assertEquals('USD', $baseAmount['currency']);
     }
 
+    /**
+     * @magentoApiDataFixture Magento/Customer/_files/customer.php
+     * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
+     * @magentoApiDataFixture Magento/GraphQl/Quote/_files/customer/create_empty_cart.php
+     * @magentoApiDataFixture Magento/GraphQl/Quote/_files/add_simple_product.php
+     * @magentoApiDataFixture Magento/GraphQl/Quote/_files/set_new_shipping_address.php
+     */
+    public function testGetSelectedShippingMethodBeforeSet()
+    {
+        $maskedQuoteId = $this->getMaskedQuoteIdByReservedOrderId->execute('test_quote');
+
+        $query = $this->getQuery($maskedQuoteId);
+        $response = $this->graphQlQuery($query, [], '', $this->getHeaderMap());
+
+        self::assertArrayHasKey('cart', $response);
+        self::assertArrayHasKey('shipping_addresses', $response['cart']);
+        self::assertCount(1, $response['cart']['shipping_addresses']);
+
+        $shippingAddress = current($response['cart']['shipping_addresses']);
+        self::assertArrayHasKey('selected_shipping_method', $shippingAddress);
+
+        self::assertArrayHasKey('carrier_code', $shippingAddress['selected_shipping_method']);
+        self::assertNull($shippingAddress['selected_shipping_method']['carrier_code']);
+
+        self::assertArrayHasKey('method_code', $shippingAddress['selected_shipping_method']);
+        self::assertNull($shippingAddress['selected_shipping_method']['method_code']);
+
+        self::assertArrayHasKey('carrier_title', $shippingAddress['selected_shipping_method']);
+        self::assertNull($shippingAddress['selected_shipping_method']['carrier_title']);
+
+        self::assertArrayHasKey('method_title', $shippingAddress['selected_shipping_method']);
+        self::assertNull($shippingAddress['selected_shipping_method']['method_title']);
+    }
+
     /**
      * _security
      * @magentoApiDataFixture Magento/Customer/_files/customer.php
@@ -193,6 +227,10 @@ private function getQuery(string $maskedQuoteId): string
 {
   cart(cart_id: "$maskedQuoteId") {
     shipping_addresses {
+      available_shipping_methods {
+        carrier_code
+        method_code
+      }
       selected_shipping_method {
         carrier_code
         method_code

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

@@ -80,6 +80,39 @@ public function testGetSelectedShippingMethod()
         self::assertEquals('USD', $baseAmount['currency']);
     }
 
+    /**
+     * @magentoApiDataFixture Magento/GraphQl/Catalog/_files/simple_product.php
+     * @magentoApiDataFixture Magento/GraphQl/Quote/_files/guest/create_empty_cart.php
+     * @magentoApiDataFixture Magento/GraphQl/Quote/_files/add_simple_product.php
+     * @magentoApiDataFixture Magento/GraphQl/Quote/_files/set_new_shipping_address.php
+     */
+    public function testGetSelectedShippingMethodBeforeSet()
+    {
+        $maskedQuoteId = $this->getMaskedQuoteIdByReservedOrderId->execute('test_quote');
+
+        $query = $this->getQuery($maskedQuoteId);
+        $response = $this->graphQlQuery($query);
+
+        self::assertArrayHasKey('cart', $response);
+        self::assertArrayHasKey('shipping_addresses', $response['cart']);
+        self::assertCount(1, $response['cart']['shipping_addresses']);
+
+        $shippingAddress = current($response['cart']['shipping_addresses']);
+        self::assertArrayHasKey('selected_shipping_method', $shippingAddress);
+
+        self::assertArrayHasKey('carrier_code', $shippingAddress['selected_shipping_method']);
+        self::assertNull($shippingAddress['selected_shipping_method']['carrier_code']);
+
+        self::assertArrayHasKey('method_code', $shippingAddress['selected_shipping_method']);
+        self::assertNull($shippingAddress['selected_shipping_method']['method_code']);
+
+        self::assertArrayHasKey('carrier_title', $shippingAddress['selected_shipping_method']);
+        self::assertNull($shippingAddress['selected_shipping_method']['carrier_title']);
+
+        self::assertArrayHasKey('method_title', $shippingAddress['selected_shipping_method']);
+        self::assertNull($shippingAddress['selected_shipping_method']['method_title']);
+    }
+
     /**
      * _security
      * @magentoApiDataFixture Magento/Customer/_files/customer.php
@@ -149,6 +182,10 @@ private function getQuery(string $maskedQuoteId): string
 {
   cart(cart_id: "$maskedQuoteId") {
     shipping_addresses {
+      available_shipping_methods {
+        carrier_code
+        method_code
+      }
       selected_shipping_method {
         carrier_code
         method_code