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

[sentry]

Sentry Issue: FLAGSMITH-API-5GZ

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"]