fix(eventstore): Handle Group.DoesNotExist in Event.group property

[sentry]

This PR addresses an issue where accessing `event.group` could lead to a `Group.DoesNotExist` exception, causing 500 errors on endpoints like `/api/0/organizations/{organization_id_or_slug}/events/{project_id_or_slug}:{event_id}/`.

The root cause was identified in src/sentry/services/eventstore/models.py within the Event.group property. The property directly calls Group.objects.get(id=self.group_id) without any exception handling. If the referenced Group object had been deleted or was not yet replicated, this call would raise Group.DoesNotExist, bubbling up and crashing the request.

The solution implemented wraps the Group.objects.get() call in a try-except Group.DoesNotExist block. If the Group is not found, None is returned and cached for subsequent accesses. This aligns the behavior with cases where self.group_id is None, ensuring that a missing group is gracefully handled without raising an exception. This makes the Event.group property more robust and prevents unexpected 500 errors.

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Fixes SENTRY-3ZZB