Handle index processing event case where an edge exists but entity is missing.

In the event an edge exists in graph and the entity has been deleted, ensure this is handled as OK when processing the index event in AmazonAsyncEventService. Currently it logs an exception because when it tries to fetch the entity data it doesn't exist and therefore 0 messages are returned for an event that should have returned 1 message.

Ideally, this should trigger a graph node compaction (read repair) to remove that edge.