[AKS] fix: aks cluster create issue

[andyzhangx]

Description
fix: aks cluster create issue mentioned here: #14688 (comment)

`>az aks create -g vsoto-aks-aad-test-rg -n vsoto-aks-aad-test --enable-aad --aad-admin-group-object-id <>
Finished service principal creation[##################################] 100.0000%Property id '' at path 'properties.diskEncryptionSetID' is invalid. Expect fully qualified resource Id that start with '/subscriptions/{subscriptionId}' or '/providers/{resourceProviderNamespace}/'.

az --version
azure-cli 2.11.0

core 2.11.0
telemetry 1.0.5

Extensions:
azure-devops 0.11.0`

Testing Guide

History Notes

[Component Name 1] BREAKING CHANGE: az command a: Make some customer-facing breaking change.
[Component Name 2] az command b: Add some customer-facing feature.

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cc @vinisoto @myronfanqiu

[saiyan86]

/lgtm. @andyzhangx Thanks for the fix!

[yungezz]

hi @arrownj can this PR be merged now?