*** Original commit ***
Prevent creation of new sessions if the Timeline is empty.
We currently create sessions based on the placeholder window
index. This shouldn't be needed as we now set a non-empty
timeline as soon as the first MediaItem is added to the playlist.
Once this check is part of the session manager, we can also remove
the equivalent workarounds from the various code integrations.
***
PiperOrigin-RevId: 369452067
