Client Receives Incorrect Parent on Server Detachment if AttachParent Is Null

User Description:

In USceneComponent::PreNetReceive the old (in this case, non-null) AttachParent is being cached into NetOldAttachParent. In USceneComponent::PostRepNotifies, AttachParent and NetOldAttachParent are exchanged before then running AttachToComponent on NetOldAttachParent.

After the exchange, and before the AttachToComponent, this means that AttachParent is its original, pre-replicated, value (which in this case is not null), and NetOldAttachParent now has the newly replicated value. In the case of a non-null parent being replicated, the value of NetOldAttachParent goes through AttachToComponent, causing AttachParent to be overwritten, which means it then matches the host. BUT, when it's null, AttachToComponent will basically just ignore it, which means AttachParent will stay the original, pre-replicated value (which in this case, is non-null).

1. Download and open the project: [Link Removed]
2. Play in New Window with 2 Players
3. Walk over to the "Incorrect Parent on Client on Detach" example

Result: When the server detaches from its parent, the client is offset and remains attached to the original parent.

Expected: The client would detach from the parent

Fix UE-51650

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