The changes contained in the RS have been sent by the DS that connected to it when this DS compared its own state with the state of the RS, and decided to update it with its 
missing changes contained in the ds-sync-hist attribute of the changed entries.

As described at 
https://www.opends.org/wiki/page/EffectsOfReplicationServerCrashes 
the case where a DS updates an empty RS is not a realistic case because it is valid only in a corner case .. and could lead to an update of a massive unexpected number of 
changes sent to the RS in all other cases.

So the fix here consists in testing in the DS that the RS is not empty before deciding to send the missing changes.