Pagination token may be missing when joining a room over federation

[matrixbot]

This issue has been migrated from #3915.

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This is the synapse side of element-hq/element-web#7335.

Looks like there may be some bug where your /sync on joining a remote room does not contain a pagination token, so you can't paginate backwards. If you later re-/sync you get a valid pagination token. It feels like there's a if len(local dag) == 0: pagination_token = None somewhere in synapse.