Prevent undefined index warning before setting shipping method

[pmclain]

Description (*)

list could 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.

Fixed Issues (if relevant)

1. Notice: Undefined offset: 1 in app/code/Magento/QuoteGraphQl/Model/Resolver/ShippingAddress/SelectedShippingMethod.php on line 35 #657

Manual testing scenarios (*)

Reliably reproducing the original error depends on the order of the field properties in the query, available_shipping_methods MUST precede selected_shipping_method.

1. Create empty cart
2. Add simple product to cart
3. Set shipping method on cart
4. Request available rates and selected shipping method from cart

{
  cart(cart_id: "$maskedQuoteId") {
    shipping_addresses {
      available_shipping_methods {
        carrier_code
        method_code
      }
      selected_shipping_method {
        carrier_code
        method_code
        carrier_title
        method_title
        amount {
            value
            currency
        }
        base_amount {
            value
            currency
        }
      }
    }
  }
}

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[TomashKhamlai]

Works great! @naydav, please review.

[ghost]

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