Partially updating a multivariate option returns 500

`PATCH /api/v1/projects/{project}/features/{feature}/mv-options/{id}/` returns a 500 when the payload leaves out `default_percentage_allocation`, which prevents updates to change only a variant key.

## Steps To Reproduce

1. Create a multivariate feature with at least one variant.
2. `PATCH .../mv-options/{id}/` with `{"feature": , "key": "hero"}`.
3. The response is 500.

## Expected behaviour

A partial update succeeds, leaving the allocation as it is.

Start by reproducing the PATCH request against /api/v1/projects/{project}/features/{feature}/mv-options/{id}/ with the feature and key fields only, then trace the API serializer and update path handling default_percentage_allocation. Done means the partial update returns successfully, preserves the existing allocation, and has coverage for the reported request.

Written by the indexing model from the issue text.