Infrequently the test flaked. Reproducable with:
```
go test go.etcd.io/etcd/tests/v3/integration --run TestFirstCommitNotification --count=500
```
The moveLeader finishes when configchange is commited by quorum.
It doesn't guarantee that the 'empty' record was committed by the new leader.
From time to time happened that appliedLeaderIndex was returning 9
(without empty entry) and the test flaked. In healthy case the
appliedIndex returned 10.
Fixed by putting kv pair after leader change. The pair is guaranteed
to be stored on index when put finishes (so the empty entry as well).