Perform nil check on old lb in azurecluster_validation.go

[nawazkh]

What type of PR is this?
/kind bug

What this PR does / why we need it:

• Thanks @willie-yao for finding the bug!
• This PR adds a nil check on the old lb in the validateAPIServerLB() function to avoid any nil pointer errors.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes # private cluster tests

Special notes for your reviewer:

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• cherry-pick candidate

Release note:

perform nil check on private lb

[nawazkh]

/cc @willie-yao

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 52.44%. Comparing base (8b8ba05) to head (3c00cc1).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
api/v1beta1/azurecluster_validation.go	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5378   +/-   ##
=======================================
  Coverage   52.44%   52.44%           
=======================================
  Files         272      272           
  Lines       29349    29349           
=======================================
  Hits        15391    15391           
  Misses      13152    13152           
  Partials      806      806           

☔ View full report in Codecov by Sentry.
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[willie-yao]

/lgtm
/test pull-cluster-api-provider-azure-e2e-optional

Will approve once double-checking the test passes

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: a9cd98bfccb1c5301722e57cda720ec8e3cf56be

[willie-yao]

/approve
/hold

It would be good to get some runs of this over the weekend, so I'll just approve and feel free to remove the hold once the test passes.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: willie-yao

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• OWNERS [willie-yao]

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[willie-yao]

/hold cancel