[release-1.19] Only update the subnet spec if status ASO CIDRs not empty

[k8s-infra-cherrypick-robot]

This is an automated cherry-pick of #5584

/assign nawazkh

Subnet spec should not be updated if status ASO CIDRs are empty.

[k8s-ci-robot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.55%. Comparing base (1d0349e) to head (283c981).
Report is 1 commits behind head on release-1.19.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-1.19    #5591      +/-   ##
================================================
+ Coverage         52.54%   52.55%   +0.01%     
================================================
  Files               272      272              
  Lines             29474    29478       +4     
================================================
+ Hits              15486    15492       +6     
+ Misses            13186    13184       -2     
  Partials            802      802              

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[k8s-ci-robot]

@k8s-infra-cherrypick-robot: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cluster-api-provider-azure-apiversion-upgrade-v1beta1	283c981	link	true	/test pull-cluster-api-provider-azure-apiversion-upgrade-v1beta1

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