[release-1.11] Fix nil pointer exception when deleting MachinePools

[k8s-infra-cherrypick-robot]

This is an automated cherry-pick of #4019

/assign nojnhuh

NONE

[nojnhuh]

/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: e386a5357c7a99047fd2e4ef29ed9197d1da9630

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nojnhuh

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[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.02% ⚠️

Comparison is base (cde859a) 56.57% compared to head (cca4183) 56.56%.
Report is 1 commits behind head on release-1.11.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-1.11    #4025      +/-   ##
================================================
- Coverage         56.57%   56.56%   -0.02%     
================================================
  Files               187      187              
  Lines             19126    19130       +4     
================================================
  Hits              10821    10821              
- Misses             7674     7678       +4     
  Partials            631      631              

Files Changed	Coverage Δ
azure/scope/machinepool.go	29.18% <0.00%> (-0.21%)	⬇️

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