Should make argocd-notifications-secret to an optional argument

[kumo-rn5s]

Summary

For webhooks that do not use secrets(like discord),
the notification controller should not detect the existence of argocd-notifications-secret

Diagnostics

My environment:
EKS 1.22
Argocd v2.4.11+3d9e9f2
Argocd notifications v1.2.1

This is the log without argocd-notifications-secret applied

time="2022-09-08T14:01:23Z" level=info msg="Start processing" resource=argocd/app
time="2022-09-08T14:01:23Z" level=error msg="Failed to process: secret \"argocd-notifications-secret\" not found" resource=argocd/app

After adding a empty secret

apiVersion: v1
kind: Secret
metadata:
  name: argocd-notifications-secret
type: Opaque

The controller is available immediately

time="2022-09-08T14:01:23Z" level=info msg="Start processing" resource=argocd/app
time="2022-09-08T14:01:23Z" level=info msg="Processing skipped: sync status out of date" resource=argocd/app

I have just used argocd for a short time, so please correct me if my description is technically incorrect.

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