Insufficient validation in ` /api/v1/projects/{project_pk}/features/{feature_pk}/mv-options/`

Sentry Issue: [FLAGSMITH-API-5GZ](https://flagsmith.sentry.io/issues/7223700739/?referrer=github_integration)

```
KeyError: 'default_percentage_allocation'
(9 additional frame(s) were not displayed)
...
File "features/multivariate/views.py", line 89, in create
return super().create(request, *args, **kwargs)
File "features/multivariate/serializers.py", line 68, in validate
total_sibling_percentage_allocation + attrs["default_percentage_allocation"]
```