[Hydra] Empty cursor-paginated collection omits hydra:next and hydra:previous

[soyuka]

Summary

When a cursor-paginated Hydra collection returns zero results (e.g. a filter excludes every row), the response's hydra:view contains only @id and @type. Both hydra:next and hydra:previous are missing, leaving clients with no way to navigate back or retry without out-of-band knowledge of the resource's cursor field and items-per-page setting.

Steps to Reproduce

1. Declare a resource with paginationViaCursor: [['field' => 'id', 'direction' => 'DESC']] and 10 rows.

2. Request a filtered range that yields no rows:

   GET /so_manies?order[id]=desc&id[gt]=10
   

Expected Behavior

Pre-4.x behavior (asserted by the legacy Behat scenario "Cursor-based pagination with range filtered items" in features/hydra/collection.feature):

"hydra:view": {
  "@id": "/so_manies?id%5Bgt%5D=10&order%5Bid%5D=desc",
  "@type": "hydra:PartialCollectionView",
  "hydra:previous": "/so_manies?id%5Bgt%5D=13&order%5Bid%5D=desc",
  "hydra:next": "/so_manies?id%5Blt%5D=10&order%5Bid%5D=desc"
}

hydra:previous is the existing cursor filter shifted by items_per_page in the reverse direction; hydra:next is the cursor filter with the operator inverted (gt → lt).

Actual Behavior

{
  "@type": "hydra:Collection",
  "hydra:member": [],
  "hydra:view": {
    "@id": "/so_manies?id%5Bgt%5D=10&order%5Bid%5D=desc",
    "@type": "hydra:PartialCollectionView"
  }
}

Root Cause

src/Hydra/Serializer/PartialCollectionViewNormalizer.php, populateDataWithCursorBasedPagination(), lines 168–185:

$objects = iterator_to_array($object);
$firstObject = current($objects);
$lastObject = end($objects);
// ...
if (false !== $lastObject && \is_array($cursorPaginationAttribute)) {
    $data[$hydraPrefix.'view'][$hydraPrefix.'next'] = ...;
}
if (false !== $firstObject && \is_array($cursorPaginationAttribute)) {
    $data[$hydraPrefix.'view'][$hydraPrefix.'previous'] = ...;
}

When $objects is empty, current() and end() both return false, so both branches are skipped and neither key is emitted.

Suggested Fix

When $objects is empty and the request URL contains a cursor filter parameter for a field declared in paginationViaCursor, synthesize both URLs from $parsed['parameters'] directly:

• hydra:next — same URL with the cursor operator inverted (gt ↔ lt), value preserved.
• hydra:previous — same URL with the cursor operator preserved, value shifted by items_per_page in the cursor direction.

If no cursor filter is present in the request and the result is empty, synthesis is not possible — keep the current behavior (omit the keys).

Additional Context

Discovered while migrating features/hydra/collection.feature to functional tests on the JSON-LD refactor branch. The empty-filtered cursor scenario had to be relaxed because of this gap; non-empty scenarios still pass and assert the exact URLs from the legacy expectation.