Use more accurate rollback message during sync if no item was rolled back

Problem/Motivation

If running drush migrate:import with the --sync flag, while nothing needs to be synced (removed on the destination), the drush command will still output "Rolled back 1 item - done with '@name", because 0 is considered a singular value.
This is confusing, as one may think that an item got deleted, while none was.

Steps to reproduce

Consecutively run drush migrate:import --all and drush migrate:import --all --sync . Drush will output "Rolled back 1 item - done with '@name'", while nothing has changed.

Proposed resolution

Either print "No item has been rolled back - done with '@name'" (approach chosen in MR), or do not print anything if nothing got removed.

Remaining tasks

Review MR