nullable parts in body prototypes (#12935)

Co-authored-by: DrSmugleaf <DrSmugleaf@users.noreply.github.com>

Content.Shared/Body/Systems/SharedBodySystem.Body.cs

@@ -88,6 +88,9 @@ public partial class SharedBodySystem
         foreach (var connection in connections)
         {
             var childSlot = prototype.Slots[connection];
+            if (childSlot.Part == null)
+                continue;
+
             var childPart = Spawn(childSlot.Part, coordinates);
             var childPartComponent = Comp<BodyPartComponent>(childPart);
             var slot = CreatePartSlot(connection, parent.Owner, childPartComponent.PartType, parent);