Prevent Error When Getting Payment Method Before Setting

[pmclain]

Description (*)

Calling Magento\Quote\Model\Quote\Payment::getMethodInstance
throws an exception when the method is not set. This would trigger and error
when requsting the selected_payment_method cart property. This commit returns
an empty string instead.

Fixed Issues (if relevant)

1. Internal server error "The payment method you requested is not available." for empty guest cart #667

Manual testing scenarios (*)

1. Create empty cart

mutation {
   createEmptyCart
}

2. Query cart for selected payment method

query getCart($cart_id:String!) {
  cart(cart_id:$cart_id) {
    selected_payment_method {
      code
      title
    }
  }
}

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)

[naydav]

@pmclain Good point!

@TomashKhamlai, please, check the same behavior with shipping methods
thanks

[pmclain]

This should resolve #667

[ghost]

Hi @pmclain, thank you for your contribution!
Please, complete Contribution Survey, it will take less than a minute.
Your feedback will help us to improve contribution process.