XDCR - may raise empty backfill that leads to TopTask is empty messages

When source/target manifest move, XDCR tries to diff them and create backfill requests. There is a bug where if there is nothing to backfill because implicit or explicit mapping show no differences, that it'll raise an empty backfill request that will cause backfill spec to be created.

The backfill pipeline will start but notice that there's no task to start and will spam the UI with error messages as shown in MB-48676

For this to happen, rebalance has to be happening where VBs are being added to a node, and manifest changes on the target need to happen at the same time so that XDCR will notice when it pulls the new target manifest at the same window.