fix(uptime): Prevent Environment.DoesNotExist on monitor update

[sentry]

This PR addresses an Environment.DoesNotExist error that occurred when attempting to update an uptime monitor that was originally created without an associated environment, and the update payload also omitted the environment field.

Root Cause:
The UptimeMonitorValidator.update() method's logic for handling the environment field had a flaw. If the environment field was not present in the update data payload, it would fall into an else branch that attempted to retrieve the existing environment using Environment.objects.get(name=instance.config["environment"]). For monitors created without an environment, instance.config["environment"] is None, leading to Environment.objects.get(name=None) which raises Environment.DoesNotExist.

Fix:
The update() method's environment handling has been modified to mirror the robust logic already present in the create() method. It now explicitly checks if the environment name (either from the update payload or the existing monitor's configuration) is None. If it is None, the environment variable is set to None directly, bypassing the database query. Otherwise, Environment.get_or_create() is used to fetch or create the environment.

Regression Test:
A new test case, test_update_without_environment_when_monitor_has_none, has been added to tests/sentry/uptime/endpoints/test_project_uptime_alert_details.py to ensure this specific scenario is covered and remains fixed. The test was also updated to correctly simulate a monitor created without an environment by explicitly setting detector.config["environment"] = None after creation.

Fixes SENTRY-4MYH

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Fixes SENTRY-4MYH

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