Fix refreshing multiple times with no new data not working

When the requested range has no results, no pagination data is returned,
so the existing `newer` request range is replaced with nil. As there
was no new data, the existing request range is still correct and should
not be replaced.

Fixes #75

Tusker/Screens/Notifications/NotificationsTableViewController.swift

@@ -208,8 +208,10 @@ class NotificationsTableViewController: EnhancedTableViewController {
             MastodonCache.addAll(statuses: newNotifications.compactMap { $0.status })
             MastodonCache.addAll(accounts: newNotifications.map { $0.account })
 
-            self.newer = pagination?.newer
-            
+            if let newer = pagination?.newer {
+                self.newer = newer
+            }
+
             DispatchQueue.main.async {
                 self.refreshControl?.endRefreshing()
                 

Tusker/Screens/Profile/ProfileTableViewController.swift

@@ -218,7 +218,9 @@ class ProfileTableViewController: EnhancedTableViewController {
             MastodonCache.addAll(statuses: newStatuses)
             self.timelineSegments[0].insert(contentsOf: newStatuses.map { ($0.id, .unknown) }, at: 0)
 
-            self.newer = pagination?.newer
+            if let newer = pagination?.newer {
+                self.newer = newer
+            }
 
             DispatchQueue.main.async {
                 self.refreshControl?.endRefreshing()

Tusker/Screens/Timeline/TimelineTableViewController.swift

@@ -127,9 +127,14 @@ class TimelineTableViewController: EnhancedTableViewController {
         let request = MastodonController.client.getStatuses(timeline: timeline, range: newer)
         MastodonController.client.run(request) { response in
             guard case let .success(newStatuses, pagination) = response else { fatalError() }
-            self.newer = pagination?.newer
+            
             MastodonCache.addAll(statuses: newStatuses)
             self.timelineSegments[0].insert(contentsOf: newStatuses.map { ($0.id, .unknown) }, at: 0)
+            
+            if let newer = pagination?.newer {
+                self.newer = newer
+            }
+            
             DispatchQueue.main.async {
                 self.refreshControl?.endRefreshing()