NoneType error in FeatureState.get_create_log_message when environment default is missing

`AttributeError: 'NoneType' object has no attribute 'enabled'` in `features/models.py:941` — the `get_create_log_message` method calls `self.get_environment_default()` which returns `None`, then tries to access `.enabled` on it.

Sentry Issue: FLAGSMITH-API-5JG — all from the same trace during a bulk operation on project "snapshot-hdf1".

## Root Cause

At `features/models.py:941`:
```python
and self.enabled == self.get_environment_default().enabled # type: ignore[union-attr]
```

`get_environment_default()` returns `None` when the environment default `FeatureState` no longer exists (likely deleted concurrently). The existing `# type: ignore[union-attr]` comment acknowledges this possibility but doesn't handle it.

## Suggested Fix

Guard against `None`:
```python
env_default = self.get_environment_default()
if env_default is not None and self.enabled == env_default.enabled:
```

Or skip audit log creation when the environment default is missing.