fix: nil dereference in GetNodeStatus of cashu backend

[elnosh]

app crashes if tries Get Node Status for cashu backend.

Currently GetNodeStatus returns nil and then it tries to dereference that here https://github.com/getAlby/hub/blob/master/wails/wails_handlers.go#L573 causing it to crash.

Returning an empty object rather than nil.

[rolznz]

utACK

[rolznz]

Edit: actually there are multiple LNClients which return nil here and the method allows returning nil. I think we should actually check in the wails handler.

if (nodeStatus == nil) {
  return WailsRequestRouterResponse{Body: nil, Error: ""}
}
return WailsRequestRouterResponse{Body: *nodeStatus, Error: ""}

[elnosh]

made change to do nil check in the handler

[rolznz]

Thanks @elnosh !