feat: display order shippingMethod if there is no checkoutModules

[huextrat]

Description

This change makes it possible to display a shipping method that is available in the clientSession.order.shipping if checkoutModules is nil.

This is useful in cases where only one delivery method is available and therefore no change of shipping method is possible by the user. It's also useful to display the shippingMethod very easily just from the clientSession.

A great use case for this, is a payment without the need of the shippingAddress (so it's no EC). In that case, the shippingMethod price should not change it's already calculated and there is no need to have checkoutModules in that case.

I would have liked to update the example, but it doesn't seem to work with shippingMethod.

This should be non-breaking as checkoutModules always has priority and this change is like a fallback.

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