jottacloud: fix traditional authentication failing with invalid_client

Configuring traditional authentication set client_id but kept any
existing client_secret, for example one left in the config from legacy
authentication with a machine specific API key. Whitelabel services
that have moved to an Ory Hydra based identity provider, such as elkjop
and elko, reject a client secret for their public clients, so the token
exchange failed with:

  invalid_client: The OAuth 2.0 Client supports client authentication
  method 'none', but method 'client_secret_post' was requested.

Clear client_secret, as standard authentication already does.

backend/jottacloud/jottacloud.go

@@ -258,6 +258,7 @@ service, for the official service on https://www.jottacloud.com/web/secure.`)
 		}
 		m.Set("configVersion", fmt.Sprint(configVersion))
 		m.Set(configClientID, service.clientID)
+		m.Set(configClientSecret, "")
 		m.Set(configTokenURL, wellKnown.TokenEndpoint)
 		return oauthutil.ConfigOut("choose_device", &oauthutil.Options{
 			OAuth2Config: &oauthutil.Config{