Error if TELEGRAM_DISABLE_NOTIFICATION_SEVERITY is missing

[zoifar]

Issue Summary
ERROR: argument of type 'NoneType' is not iterable if TELEGRAM_DISABLE_NOTIFICATION_SEVERITY is missing in alertad.conf for telegram plugin

Environment

• OS: Linux

• API version: 8.5.3

• Deployment: self-hosted

• For self-hosted, WSGI environment: nginx

• Database: Postgres

• Server config:
  Auth enabled? Yes
  Auth provider? Basic
  Customer views? No

• web UI version: 8.5.3

• CLI version: 8.5.3

To Reproduce
Steps to reproduce the behavior:

1. alerta --endpoint-url http://alerta_host/api send -r resourcetest919 -e eventtest -E Production -S Website -s Major -t "Web server is down." -v valuetest
2. alerta --endpoint-url http://alerta_host/api send -r resourcetest919 -e eventtest -E Production -S Website -s ok -t "Web server is down." -v valuetest
3. See error
   ERROR: argument of type 'NoneType' is not iterable