Cleanup: Idle activity now reuses itself instead of returning null (causing another Idle object to be created)

OpenRA.Game/Traits/Activities/Idle.cs

@@ -12,6 +12,6 @@ namespace OpenRA.Traits.Activities
 {
 	public class Idle : CancelableActivity
 	{
-		public override IActivity Tick(Actor self) { return NextActivity; }
+		public override IActivity Tick(Actor self) { return NextActivity ?? this; }
 	}
 }