[AKS] az aks: Fix monitoring addon and agentpool NoneType errors.

[JunSun17]

---

This checklist is used to make sure that common guidelines for a pull request are followed.

General Guidelines

• Have you run azdev style <YOUR_EXT> locally? (pip install azdev required)
• Have you run python scripts/ci/test_index.py -q locally?

For new extensions:

• My extension description/summary conforms to the Extension Summary Guidelines.

[azuresdkci]

If this PR is for a new extension or change to an existing extension, use the following to try out the changes in this PR:

docker run -it microsoft/azure-cli:latest
export EXT=<NAME>
pip install --upgrade --target ~/.azure/cliextensions/$EXT "git+https://github.com/JunSun17/azure-cli-extensions.git@jusu-fix-nonetype#subdirectory=src/$EXT&egg=$EXT"

[xizha162]

/LGTM

[JunSun17]

@zikalino Can you help to approve and merge this change, thanks!

[jluk]

@qwordy can you merge this as well? It is the same fix as the one merged below, but for the preview extension.

Azure/azure-cli#12250