fix: Sort items correctly when empty custom group is present #2268

[marek-mihok]

The PR fulfills these requirements: (check all the apply)

• [x] It's submitted to the main branch.
• [x] When resolving a specific issue, it's referenced in the PR's title (e.g. feat: Add a button #xxx, where "xxx" is the issue number).
• [x] When resolving a specific issue, the PR description includes Closes #xxx, where "xxx" is the issue number.
• [x] If changes were made to ui folder, unit tests (make test) still pass.
• [x] New/updated tests are included

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The problem was in reducer inside onSortChange function

setFilteredItems(filteredItems => [...groups]
     // sorts groups by startIndex to match its order in filteredItems
     .sort((group1, group2) => group1.startIndex - group2.startIndex)
     .reduce((acc, group) => [...acc, ...filteredItems.slice(group.startIndex, acc.length + group.count).sort(sortingF(column, sortAsc))],
     [] as any[]) || [])

adding acc.length + group.count filtered items into the final array even in case of empty group present (group.count equal to 0) but acc.length having non-zero value most of the time.

Fixed by conditional adding of items:

group.count
       ? [...acc, ...filteredItems.slice(group.startIndex, acc.length + group.count).sort(sortingF(column, sortAsc))]
       : acc

Closes #2268