[14.0][FIX] shopinvader: return of dispatch should be something else than dict

[acsonefho]

In some case (using shopinvader/payment/adyen/webhook for example), the return value is a str (dict is expected). That raise an exception.

The question is:

• Adyen expect this value? If yes, so this PR should be merged;
• If no, we have to fix into invader_payment_adyen and return a dict.

https://github.com/shopinvader/odoo-shopinvader-payment/blob/14.0/invader_payment_adyen/services/payment_adyen.py#L575