Update session index cleanup to check for missing shards

[SiddharthMantri]

Closes #205146

Summary

We run a session clean up task that opens a point in time query to try and delete any older sessions in the session index. We've noticed that this task fails quite often with the same error no_shard_available_action_exception. On investigating, it's possible that the point in time query is opened when there are no shards available for that index. This PR fixes that by first checking if the shards are available and bails if not present - allowing the task to be tried again in the next run of the task manager.

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[SiddharthMantri]

@elasticmachine merge upstream

[SiddharthMantri]

I wonder why this file is here? 🤔