Dummy 'summary' message for Pagerduty integration discussion

**Slack thread:** https://grafana.slack.com/archives/C043CEX9MBK/p1689199648823409

**Summary:**
I ran into a situation recently where the Pagerduty-Grafana integration was failing to send 'alert resolved' notifications to a Pagerduty contact point after the alert resolved. The problem was due to a golang template for the 'summary' section of the contact point being wrapped in an {{ if .Alerts.Firing }} tag (which of course would result in an empty summary upon resolution of an alert). Pagerduty was responding with a 400 every time this empty summary came through, resulting in a failure to auto-resolve a page.

I was asked by @yuri-tceretian to generate an issue so ya'll could discuss the addition of possible "protect the user from their own misconfiguration" dummy text upon empty summary.

**My own thoughts:**
The addition of golang templating to grafana alerts in grafana 9 looks really powerful, but it's still pretty complex and not entirely transparent to the userbase yet... I'm not sure I (as a user) would appreciate the addition of an extra 'sane default' in an ecosystem where there's already a lot that can go wrong and not quite as much transparency on the alert compilation process...

... but you guys are the experts, go get 'em. :)

Start with the linked Slack thread and the PagerDuty contact point handling described in the issue; no source file or test is named. Determine whether the project wants to handle an empty templated summary, then document the agreed behavior and identify the relevant implementation and test locations before proceeding.

Written by the indexing model from the issue text.