Profile save returns 502 when optional fields are null

When saving a user profile via `PUT /v1/users/{user_id}`, the backend returns a **502 Bad Gateway** if optional fields are sent as `null`. According to the OpenAPI spec (`/v1/openapi.json`), all fields in `UserProfileUpdateRequest` are optional and accept `null` via `anyOf [type, null]`. ### Steps to reproduce 1. Register a new user 2. Activate and verify the account 3. Save the profile with only `first_name`, `last_name`, `display_name`, and `lang` populated — all other fields sent as `null` ### Expected 200 OK (the spec says all fields are optional/nullable) ### Actual 502 Bad Gateway — no error details returned, no indication of which field is invalid ### Additional context - The frontend BFF converts empty form fields to `null` via `stripEmptyStrings()` before forwarding to the backend - `GET /v1/users/me`, `PATCH .../activate`, and `PATCH .../verify` all work fine for the same user - Saving works when all fields are manually filled in via the UI - If certain fields are actually required, the backend should return a **422** with field-level errors, not a 502 - Discovered during E2E test setup (OP#2762)