[AKS] CLI should tolerate empty kubeconfig

[matthchr]

Description
Fix bug #13846

Testing Guide
Run az aks get-credentials --resource-group <rg> --name <name> on an AKS cluster while your local kubeconfig is empty:

$ kubectl config view
apiVersion: v1
clusters: null
contexts: null
current-context: ""
kind: Config
preferences: {}
users: null

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[matthchr]

Not sure what the right component prefix is supposed to be here...

[arrownj]

existing[key] == None / 0 / [] / "" / {} will all trigger this condition, is this the expected behavior ?

[matthchr]

I'm not an expert on what "valid" vs "invalid" kubeconfigs look like, so I don't know for sure.

I think at the very least a missing key should be treated the same as an empty key (before, a missing key triggered an exception).

I was matching the line above on 1214 which was was doing the same thing (not checking specifically for == False or something.

Looking further on in the code, if we don't overwrite None / 0 / "" the code below will blow up I think as we'll try to iterate over it assuming that it's a dict.

[arrownj]

Add @zqingqing1 @tjprescott to review.

[zqingqing1]

lgtm.

[matthchr]

@arrownj @zqingqing1 - is there more I need to do here? If not can we merge this?