Fixup of spawnables from 5.4 to 5.5 loses custom track names

In the process of upgrading from 5.4 to 5.5.

One of our systems relies on the internal track names in the sequence to dynamically find an actor to bind the track to.

As I was testing the system in 5.5, I noticed that all our track names were reverted to the default names, and so breaking our system.

I managed to track this issue to ULevelSequence::ConvertOldSpawnables() and the call in it to MovieScene->;AddPossessable(NewPossessable, PossessableBinding);.

Inside AddPossessable there's this loop:|
<pre class="ql-syntax">for (auto Track : NewBinding.GetTracks())

</pre>

Since the rename is being called with nullptr for the name, the existing track name is overwritten with the default track name.

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