Display "Not shared" indicator for RLS-restricted relation fields (#17713)

## Context
Previously, if for example a Person had a companyId but the company
relation was null due to RLS, the company column appeared empty. Now it
displays ForbiddenFieldDisplay to indicate the relation exists but is
inaccessible.

When RLS restricts access to a related record, the frontend now shows a
"Not shared" indicator (with lock icon) instead of an empty field.

<img width="1275" height="399" alt="Screenshot 2026-02-04 at 15 37 53"
src="https://github.com/user-attachments/assets/870306f8-f811-4dc0-b23b-a33e89043a37"
/>

packages/twenty-server/src/engine/api/graphql/graphql-query-runner/helpers/process-nested-relations-v2.helper.ts

@@ -417,9 +417,6 @@ export class ProcessNestedRelationsV2Helper {
           (rel) => rel[joinField] === item.id,
         );
       } else {
-        if (relationResults.length === 0) {
-          item[`${sourceFieldName}Id`] = null;
-        }
         item[sourceFieldName] =
           relationResults.find(
             (rel) => rel.id === item[`${sourceFieldName}Id`],