fix(monitor): set monitors to nil after closing them (backport #3388)

[mergify]

Which issue(s) this PR fixes:

Issue longhorn/longhorn#10035

What this PR does / why we need it:

Special notes for your reviewer:

Additional documentation or context

---

This is an automatic backport of pull request #3388 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of c921e2b has failed:

On branch mergify/bp/v1.6.x/pr-3388
Your branch is up to date with 'origin/v1.6.x'.

You are currently cherry-picking commit c921e2b6.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   controller/monitor/disk_monitor.go

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   controller/monitor/environment_check_monitor.go

Cherry-pick of a65a6ce has failed:

On branch mergify/bp/v1.6.x/pr-3388
Your branch is ahead of 'origin/v1.6.x' by 2 commits.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit a65a6cea.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   controller/monitor/environment_check_monitor.go
	modified:   controller/monitor/fake_disk_monitor.go
	new file:   controller/monitor/fake_environment_check_monitor.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   controller/monitor/disk_monitor.go
	both modified:   controller/node_controller.go
	both modified:   controller/node_controller_test.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[derekbit]

Mistakenly add backport/v1.6.4 label. No need to backport the PR to v1.6.4.