MINOR: Check ELR in handleAssignReplicasToDirs

[CalvinConfluent]

The partitionChangeBuilder in handleAssignReplicasToDirs does not check
ELR. This can cause the partition clear ELR unexpectedly.

[junrao]

@CalvinConfluent : Thanks for the PR. Do we need to add a test to cover this issue?

[chia7712]

@CalvinConfluent thanks for the patch. Quick unrelated question about handleAssignReplicasToDirs: Should the election type depend on unclean.leader.election.enable here? If all ISRs are in offline directies, users would likely expect an unclean election to bring the partition back.

[CalvinConfluent]

@chia7712 I think we already did. The election type is set in generateLeaderAndIsrUpdates
https://github.com/apache/kafka/blob/trunk/metadata/src/main/java/org/apache/kafka/controller/ReplicationControlManager.java#L2028

[artemlivshits]

LGTM

[CalvinConfluent]

@chia7712 @junrao Can you help review this change? Thanks a lot!