fix: allow teamLead field to be nullable in VisitResolver

[jekabs-karklins]

Description

This PR makes the 'teamLead' field nullable in 'VisitResolver'.

Motivation and Context

In the database the team lead always exists — the column is NOT NULL. The nullability here is about the read path, not the data model: teamLead resolves through getBasic, which returns null when the requesting user lacks permission to read that user's details.

While the field was non-nullable, that null triggered GraphQL's non-null propagation: the error bubbles up to the nearest nullable ancestor, Experiment.visit, and removes the entire visit object from the response. A single unreadable team lead therefore made the visit look as though it didn't exist, which is a far larger side effect than the missing field itself.

Making the field nullable confines the loss to the field that's actually restricted.

Changes

1. Made 'teamLead' field nullable in 'VisitResolver' (apps/backend/src/resolvers/types/Visit.ts).
2. Adjusted related code in frontend components and tests to handle 'teamLead' being null (apps/e2e/cypress/e2e/visits.cy.ts, apps/frontend/src/components/experiment/ExperimentVisitsTable.tsx, apps/frontend/src/components/proposalBooking/CreateUpdateVisit.tsx).
3. Updated GraphQL queries to include 'teamLeadUserId' (apps/frontend/src/graphql/experiment/getAllExperiments.graphql, apps/frontend/src/graphql/experiment/getExperiment.graphql, apps/frontend/src/graphql/visit/fragment.visit.graphql).

How Has This Been Tested?

Fixes Jira Issue

https://jira.ess.eu//browse/

Depends On

Tests included/Docs Updated?

• I have added tests to cover my changes.
• All relevant doc has been updated