Review null response in webserver_api.get_service_pricing_plan

[pcrespov]

webserver_api.get_service_pricing_plan can return None and will fail with 500 while serializing the response since response_model=ServicePricingPlanGet,

https://github.com/ITISFoundation/osparc-simcore/blob/master/services/api-server/src/simcore_service_api_server/api/routes/solvers.py#L279

On the other hand, a succesful response from the webserver will never produce a data.None ...
https://github.com/ITISFoundation/osparc-simcore/blob/master/services/api-server/src/simcore_service_api_server/services/webserver.py#L601

Probably it was confusing how Envelope was annotated

class Envelope(BaseModel, Generic[DataT]):
    data: DataT | None = None
    error: Any | None = None