fix(environments): Handle missing default environment in event processing

[seer-by-sentry]

This PR addresses issue SENTRY-434K, where Environment.DoesNotExist errors occurred during the link_event_to_user_report pipeline step in sentry.issues.tasks.post_process.post_process_group.

The root cause was identified as event.get_environment() attempting to retrieve an Environment record with an empty string name (when an event lacked an explicit environment tag). If this default empty-string environment had never been implicitly created for a given organization, the .get() call would fail.

The solution modifies BaseEvent.get_environment() in src/sentry/services/eventstore/models.py. Instead of directly calling Environment.get_for_organization_id() which uses a .get() query, it now wraps this call in a try-except Environment.DoesNotExist block. If the environment does not exist, it falls back to Environment.objects.get_or_create(name=name, organization_id=organization_id), ensuring the default empty-string environment is created if it's missing. This makes the environment lookup more robust and prevents the DoesNotExist error.

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