Cleanup PagerdutyNotifier __init__

[DavidTraina]

• fix duplicate initialization of self.class_type and self.custom_details
• allow pagerduty_events_conn_id to be None since that's allowed in PagerdutyEventsHook

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[boring-cyborg]

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[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.