Express /api/NAME/:id/FIELD_PATH/slice endpoint errors

[yangchristian]

Repro:

• Create a test for an /api/NAME/:id/FIELD_PATH/slice endpoint, e.g.:

        const result = await fetch(urlPrefix + '/api/user/3/siblings/slice');

Result:

• TypeError: Cannot read property 'first' of undefined

Note that also the endpoint is implemented as a GET but tries to use the request body, which I don't think will actually work (prob need to turn them into query params).