Alerting: when pending period is `None`, the alert rule export doesn't set the `for` field.

### What happened? Set the pending period of the alert rule to `None`  The alert rule export does not define the `for` field.  Provisioning the alert rule fails (Tested using YAML files). ### What did you expect to happen? Include `for: 0s` in the exported config file. ### Did this work before? Don't know. ### How do we reproduce it? 1. 2. 3. ### Is the bug inside a dashboard panel? _No response_ ### Environment (with versions)? Grafana: 11.3 docker image ### Grafana platform? None ### Datasource(s)? _No response_