Always include paging_metadata in paginated list responses (#538)

The AAS Part 2 API spec requires GetPagedResult wrapper on every
list response. The previous code omitted it when all results fit
on the first page, returning a plain array that clients expecting
{"result": [...], "paging_metadata": {...}} could not parse.

Fixes #519

Author: zrgt

server/app/interfaces/base.py

@@ -240,11 +240,7 @@ def _get_slice(cls, request: Request, iterator: Iterable[T]) -> Tuple[Iterator[T
         paginated_slice = iter(items[:limit])
         next_cursor = str(cursor + limit + 1) if has_more else None
 
-        if next_cursor is not None or cursor > 0:
-            # add metadata if cursor was present in request
-            paging_metadata = PagingMetadata(cursor=next_cursor)
-        else:
-            paging_metadata = None
+        paging_metadata = PagingMetadata(cursor=next_cursor)
         return paginated_slice, paging_metadata
 
     def handle_request(self, request: Request):