Fix external service resolver erroring when webhooks not supported
Created by: eseliger
Before, when requesting the webhook URL field, it would error out for external services for which we don't support webhooks.
Reported in Slack here: https://sourcegraph.slack.com/archives/C03CSAER9LK/p1653311222557759
Q: Should we do a patch release with this fix in? It breaks the external service config page.
Test plan
Verified manually the error Taras was seeing is no longer happening.