Cannot use predefined system node pools and custom preset while JustUseSystemNodepool set to true

[SzymonSt]

Describe the bug If JustUseSystemNodepool all the system nodepool presets are ignored.

To Reproduce Steps to reproduce the behavior:

1. Set JustUseSystemNodepool to true
2. Define your own custom preset for system nodepool or use on of (highspec, standard, costoptimized)
3. Deploy
4. Observe system nodepool being depoyed with just systemPoolBase properites

Expected behavior System nodepool is created with config from custom/preset definition.

Additional context My config:

//---------Kubernetes Construction---------
//ref: https://github.com/Azure/AKS-Construction
module aksConstruction '../../../AKS-Construction/bicep/main.bicep' = {
  name: 'aksconst-${deploymentPostfix}'
  scope: resourceGroup(aksClusterName)
  params: {
    //General
    resourceName: aksClusterName
    enableTelemetry: false
    kubernetesVersion: '1.27.3'                   //ToBeParametrized
    AksPaidSkuForSLA: true
    enable_aad: true
    enableAzureRBAC: true
    keyVaultAksCSI: true
    JustUseSystemPool: true
    //Networking
    networkPlugin: 'azure'
    networkPolicy: 'azure'
    byoAKSSubnetId: subnetResource.id
    enablePrivateCluster: true
    serviceCidr: '172.0.0.0/20'                   //ToBeParametrized
    dnsServiceIP: '172.0.0.10'                    //ToBeParametrized
    //Compute
    SystemPoolType: 'Custom'
    SystemPoolCustomPreset:{
      vmSize: 'Standard_D8ds_v5'                  //ToBeParametrized
      count: 1                                    //ToBeParametrized
      maxPods: 200                                //ToBeParametrized
      minCount: 1
      maxCount: 2                                 //ToBeParametrized
      enableAutoScaling: true                     //ToBeParametrized
      availabilityZones: [
        '1'
        '2'
        '3'
      ]
    }
  }
}

[SzymonSt]

I've linked PR with proposed solution. Thanks in advance for review of bug issue and PR.

[github-actions[bot]]

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