Fix error processor breaking checkout when a shipping request fails

[sippsolutions]

When a request fails during the shipping step (before the payment step) and does not return valid JSON, the error processor mixin fails because responseJSON is undefined:

error-processor-mixin.js:52 Uncaught TypeError: Cannot read properties of undefined (reading 'hasOwnProperty')
    at Object.<anonymous> (error-processor-mixin.js:52:51)
    at Object.process (wrapper.js:78:32)
    at Object.<anonymous> (fee-service.js:64:32)
    at fire (jquery.js:3223:50)
    at Object.fireWith [as rejectWith] (jquery.js:3353:29)
    at done (jquery.js:9629:30)
    at XMLHttpRequest.<anonymous> (jquery.js:9888:37)

Additionally, quote.paymentMethod() is not given within shipping step, which also fails even if we fix the the first issue.

This currently results in the loading screen remaining visible and prevents checkout in some cases (especially with custom functionality in the shipping step).

This issue can be resolved with minimal changes by using optional chaining: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining

[janteuber]

Hello @sippsolutions ,

Thank you so much for creating the pull request on GitHub. Our developer will review it shortly, and we will get back to you with feedback afterward.

Best regards,
Jan