Autoscaler should scale from zero when taints do not have a "value" field

When scaling from zero replicas, the cluster autoscaler can panic if there are taints on the machineset with no "value" field defined.

    

4.16/master
    

always
    

    1. create a machineset with a taint that has no value field and 0 replicas
    2. enable the cluster autoscaler
    3. force a workload to scale the tainted machineset
    

a panic like this is observed

I0325 15:36:38.314276       1 clusterapi_provider.go:68] discovered node group: MachineSet/openshift-machine-api/k8hmbsmz-c2483-9dnddr4sjc (min: 0, max: 2, replicas: 0)
panic: interface conversion: interface {} is nil, not string

goroutine 79 [running]:
k8s.io/autoscaler/cluster-autoscaler/cloudprovider/clusterapi.unstructuredToTaint(...)
	/go/src/k8s.io/autoscaler/cluster-autoscaler/cloudprovider/clusterapi/clusterapi_unstructured.go:246
k8s.io/autoscaler/cluster-autoscaler/cloudprovider/clusterapi.unstructuredScalableResource.Taints({0xc000103d40?, 0xc000121360?, 0xc002386f98?, 0x2?})
	/go/src/k8s.io/autoscaler/cluster-autoscaler/cloudprovider/clusterapi/clusterapi_unstructured.go:214 +0x8a5
k8s.io/autoscaler/cluster-autoscaler/cloudprovider/clusterapi.(*nodegroup).TemplateNodeInfo(0xc002675930)
	/go/src/k8s.io/autoscaler/cluster-autoscaler/cloudprovider/clusterapi/clusterapi_nodegroup.go:266 +0x2ea
k8s.io/autoscaler/cluster-autoscaler/core/utils.GetNodeInfoFromTemplate({0x276b230, 0xc002675930}, {0xc001bf2c00, 0x10, 0x10}, {0xc0023ffe60?, 0xc0023ffe90?})
	/go/src/k8s.io/autoscaler/cluster-autoscaler/core/utils/utils.go:41 +0x9d
k8s.io/autoscaler/cluster-autoscaler/processors/nodeinfosprovider.(*MixedTemplateNodeInfoProvider).Process(0xc00084f848, 0xc0023f7680, {0xc001dcdb00, 0x3, 0x0?}, {0xc001bf2c00, 0x10, 0x10}, {0xc0023ffe60, 0xc0023ffe90}, ...)
	/go/src/k8s.io/autoscaler/cluster-autoscaler/processors/nodeinfosprovider/mixed_nodeinfos_processor.go:155 +0x599
k8s.io/autoscaler/cluster-autoscaler/core.(*StaticAutoscaler).RunOnce(0xc000617550, {0x4?, 0x0?, 0x3a56f60?})
	/go/src/k8s.io/autoscaler/cluster-autoscaler/core/static_autoscaler.go:352 +0xcaa
main.run(0x0?, {0x2761b48, 0xc0004c04e0})
	/go/src/k8s.io/autoscaler/cluster-autoscaler/main.go:529 +0x2cd
main.main.func2({0x0?, 0x0?})
	/go/src/k8s.io/autoscaler/cluster-autoscaler/main.go:617 +0x25
created by k8s.io/client-go/tools/leaderelection.(*LeaderElector).Run
	/go/src/k8s.io/autoscaler/cluster-autoscaler/vendor/k8s.io/client-go/tools/leaderelection/leaderelection.go:213 +0x105
    

expect the machineset to scale up