fix: checkout page guest vault token error

[ghost]

1. Why is this change necessary?

• See feat: added customer vault token route #90 (comment)

There is currently a error on the checkout page if your account is a guest account:

2. What does this change do, exactly?

• Use a empty userIdToken for guest accounts in the SPBCheckoutDataService

3. Describe each step to reproduce the issue or behaviour.

• Log in as a guest account
• Continue to the checkout page

4. Please link to the relevant issues (if any).

• See feat: added customer vault token route #90
• See feat: added customer vault token route #90 (comment)

[ghost]

@mstegmeyer @FlorianKe @cyl3x Could you take a look at this?

[cyl3x]

@akf-bw I'm closing this in favour of #166, #167 and #168. After checking, the system config should also be respected and no token should be fetched. Still, thank you for raising this issue and opening a PR, I appreciate that 💙