Adds test to make sure that calling IComponent.HandleComponentState() with null states does not throw an exception.

Fixes state handling code on components to not throw exceptions when called with null components.

Content.Client/GameObjects/Components/StackComponent.cs

@@ -1,4 +1,4 @@
-using Content.Client.UserInterface;
+using Content.Client.UserInterface;
 using Content.Client.Utility;
 using Content.Shared.GameObjects.Components;
 using Robust.Client.UserInterface;
@@ -22,7 +22,8 @@ namespace Content.Client.GameObjects.Components
 
         public override void HandleComponentState(ComponentState curState, ComponentState nextState)
         {
-            var cast = (StackComponentState) curState;
+            if (!(curState is StackComponentState cast))
+                return;
 
             Count = cast.Count;
             MaxCount = cast.MaxCount;