[IMPROVEMENT] Prevent unnecessary updates of instanceManager status

[derekbit]

Is your improvement request related to a feature? Please describe (👍 if you like this request)

If the engine or replica process map from an instance manager is empty, instanceManager.status.InstanceEngines or instanceManager.status.InstanceReplicas will be a nil map.

reflect.DeepEqual function treats two maps var m1 map[string]process and m2 := map[string]process as different maps. Therefore, to prevent unnecessary updates, we must check both that the length of the maps is zero and that the maps are identical.

https://github.com/longhorn/longhorn-manager/blob/master/controller/instance_manager_controller.go#L1636-L1638

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Additional context

[longhorn-io-github-bot]

Pre Ready-For-Testing Checklist

• Where is the reproduce steps/test steps documented?
  The reproduce steps/test steps are at:

Run e2e test to check the fix doesn't introduce any side effects.

• Does the PR include the explanation for the fix or the feature?

#8420 (comment)

• Have the backend code been merged (Manager, Engine, Instance Manager, BackupStore etc) (including backport-needed/*)?
  The PR is at

longhorn/longhorn-manager#2737

• Which areas/issues this PR might have potential impacts on?
  Area: instance manager update
  Issues

[chriscchien]

Verified pass on longhorn master(longhorn-manager 1cda0e)

On daily regression, did not see outsatnding issue related to instanceManager.

In build 946 , all failed test case have found the root cause and have related ticket

• test_backup_lock_creation_during_deletion
  • [BUG][v1.6.1-rc3] test_backup_lock_creation_during_deletion failed with *.lck type 1 acquisition #8269
• test_filesystem_trim
  • [BUG] test_filesystem_trim randomly failed #5463
• test_system_backup_and_restore_volume_with_backingimage
  [BUG] System backup failed because backup creation failed. #8650
• tests.test_basic.test_backuptarget_invalid
  • test: refine test_backuptarget_invalid longhorn-tests#1941
• tests.test_csi.test_csi_encrypted_block_volume
• tests.test_csi.test_csi_encrypted_migratable_block_volume
  • [BUG] test_csi_encrypted_block_volume failed on master pipeline #8688