Prevent 400 error when using navigation buttons.

Co-authored-by: Claus Vium <cvium@users.noreply.github.com>

MediaBrowser.Model/Session/GeneralCommand.cs

@@ -14,9 +14,9 @@ public class GeneralCommand
     }
 
     [JsonConstructor]
-    public GeneralCommand(Dictionary<string, string> arguments)
+    public GeneralCommand(Dictionary<string, string>? arguments)
     {
-        Arguments = arguments;
+        Arguments = arguments ?? new Dictionary<string, string>();
     }
 
     public GeneralCommandType Name { get; set; }