Enable nullability in Content.Client (#3257)

* Enable nullability in Content.Client

* Remove #nullable enable

* Merge fixes

* Remove Debug.Assert

* Merge fixes

* Fix build

* Fix build

Content.Client/UserInterface/ActionMenuItem.cs

@@ -1,6 +1,4 @@
-#nullable enable
-
-using System;
+using System;
 using Content.Client.UserInterface.Stylesheets;
 using Content.Shared.Actions;
 using Robust.Client.UserInterface;