Make more uids nullable (#5794)

Content.Server/Stack/StackSystem.cs

@@ -42,7 +42,7 @@ namespace Content.Server.Stack
         public EntityUid? Split(EntityUid uid, int amount, EntityCoordinates spawnPosition, SharedStackComponent? stack = null)
         {
             if (!Resolve(uid, ref stack))
-                return default;
+                return null;
 
             // Get a prototype ID to spawn the new entity. Null is also valid, although it should rarely be picked...
             var prototype = _prototypeManager.TryIndex<StackPrototype>(stack.StackTypeId, out var stackType)
@@ -51,7 +51,7 @@ namespace Content.Server.Stack
 
             // Try to remove the amount of things we want to split from the original stack...
             if (!Use(uid, amount, stack))
-                return default;
+                return null;
 
             // Set the output parameter in the event instance to the newly split stack.
             var entity = Spawn(prototype, spawnPosition);