fix:Orleans Streaming

[gusuchengnan]

in [SetCursor] method, If last purged token does not exists, do not throw an exception, just start from the oldest message in cache #8863

Microsoft Reviewers: Open in CodeFlow

[ReubenBond]

@benjaminpetit PTAL and give this a more apt title for if/when we merge it

[ReubenBond]

We might not have a lastPurged token for a stream if we restarted and began reading from the last checkpoint (see @benjaminpetit's comment here https://github.com/dotnet/orleans/issues/8863#issuecomment-1998082092). I'll close this for now. Thank you for opening it!