fix: An emoty string nextMarker should be treated as end-of-paging.

The Box V2 API docs are inconsistent on this. However it is stated on
the paging docs(1):
> If this value is null or an empty string there are no more results to fetch.

So with this change empty string next_markers will end paging.

(1): https://developer.box.com/guides/api-calls/pagination/marker-based/#collections

Author: potrebic

Sources/Network/PagingIterator.swift

@@ -94,7 +94,9 @@ public class PagingIterator<Element: BoxModel> {
             nextPage = .streamPosition(nextStreamPosition)
         }
         // Handle marker based paging
-        else if let nextPageMarker = page.nextMarker {
+        // An Empty string, like a nil `nextMarker`, indicates the end has been reached.
+        // ref: https://developer.box.com/guides/api-calls/pagination/marker-based/
+        else if let nextPageMarker = page.nextMarker, !nextPageMarker.isEmpty {
             nextPage = .marker(nextPageMarker)
         }
         // Handle unexpected value with no paging information