Unable to export AKS nodepool

> ### `az feedback` auto-generates most of the information requested below, as of CLI version 2.0.62

**Related command**
works: az group export --name $RG --resource-ids /subscriptions/$SUB/resourcegroups/$RG/providers/Microsoft.ContainerService/managedClusters/$CLUSTER

does not work: az group export --name $RG --resource-ids /subscriptions/$SUB/resourcegroups/$RG/providers/Microsoft.ContainerService/managedClusters/$CLUSTER/agentPools/$POOL

**Describe the bug**
`az group export` works for the AKS cluster but fails for the agent pool. It returns an empty ARM template
```
{
"$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
"contentVersion": "1.0.0.0",
"parameters": {},
"resources": [],
"variables": {}
}
```

**To Reproduce**
See above

**Expected behavior**
`az group export` shows the config for a single nodepool.

**Environment summary**
xplat

**Additional context**
n/a