Refresh bookmarks on appear when the table's empty.

This can happen after the Core Data cache is deleted.

Source/Threads/AwfulBookmarksController.m

@@ -99,6 +99,7 @@ - (void)loadPageNum:(NSUInteger)pageNum
 - (BOOL)refreshOnAppear
 {
     if (![AwfulHTTPClient client].reachable) return NO;
+    if ([self.tableView numberOfRowsInSection:0] == 0) return YES;
     if (!self.lastRefreshDate) return YES;
     return [[NSDate date] timeIntervalSinceDate:self.lastRefreshDate] > 60 * 10;
 }