Fixed half of warnings having to do with allowing Null string objects. (#791)

src/Web/Views/Manage/ManageNavPages.cs

@@ -27,7 +27,7 @@ public static class ManageNavPages
     public static string PageNavClass(ViewContext viewContext, string page)
     {
         var activePage = viewContext.ViewData["ActivePage"] as string;
-        return string.Equals(activePage, page, StringComparison.OrdinalIgnoreCase) ? "active" : null;
+        return string.Equals(activePage, page, StringComparison.OrdinalIgnoreCase) ? "active" : string.Empty;
     }
 
     public static void AddActivePage(this ViewDataDictionary viewData, string activePage) => viewData[ActivePageKey] = activePage;