fix deleting old conversations
fix deleting old conversations

fixes CNVS-7051

This was failing if the stream item had been deleted by the periodic
sweeper.

Change-Id: I3f90511af25838bc4c9ddae63a2475b31e075383
Reviewed-on: https://gerrit.instructure.com/22561
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Jon Jensen <44f878afe53efc66b76772bd845eb65944ed8232@instructure.com>
Product-Review: Brian Palmer <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@instructure.com>
QA-Review: Brian Palmer <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@instructure.com>

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