Enable nullability in Content.Shared (#3626)

* Enable nullability in Content.Shared

* Fix null errors in server

* aye github i swear on me mom

Content.Server/GameObjects/Components/Body/MechanismComponent.cs

@@ -88,7 +88,7 @@ namespace Content.Server.GameObjects.Components.Body
             }
             else // If surgery cannot be performed, show message saying so.
             {
-                eventArgs.Target.PopupMessage(eventArgs.User,
+                eventArgs.Target?.PopupMessage(eventArgs.User,
                     Loc.GetString("You see no way to install the {0}.", Owner.Name));
             }
         }