Fixes an issue where last-heartbeat time was set to the first event's timestamp

[MichaelSnowden]

This PR fixes a bug where the first event for a pending activity would set its LastHeartbeatTime to the event time. I tested the fix by writing a test which calls the ReplicateActivityTaskStartedEvent method which had this issue, and verifies that the LastHeartbeatDetails are nil (as long as they were previously nil). The biggest risk to this change is if someone was depending on LastHeartbeatDetails on their pending activities being non-nil.