There are several situations in which it is a valid system state for
Keys to PollMessages on Domains to point to entities that don't exist,
chief amongst them when a domain has been soft-deleted. We also can't
simply null this out when soft-deleting domains, because there may be
an un-acked recurrence meaning it can't be deleted.
Given this, it doesn't make sense to check for entity existence here,
as it is not a system invariant and cannot easily be made to be so.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=122300368
56c8bb0f2a