Description of problem:

The documentation [1] for the KubeletConfig object states that the machineConfigPoolSelector will match all resources when the selector is empty and will match no resources when the selector is null. However, when testing I saw that an empty selector matches nothing. Also, 
the controller's source code [2] includes a comment that describes different behavior:

"// If a pool with a nil or empty selector creeps in, it should match nothing, not everything."



Version-Release number of selected component (if applicable):
OCP v4.5.9


How reproducible:
Always

Steps to Reproduce:
1. Create a KubeletConfig resource with an empty machineConfigPoolSelector
2. Look at the 'oc describe kubeletconfig <configname>' output.
3.

Actual results:
The kubeletconfig does not apply to any of the machineconfigpools and shows this condition:

'''
  Conditions:
    Last Transition Time:  2020-09-17T21:00:06Z
    Message:               Error: could not find any MachineConfigPool set for KubeletConfig
    Status:                False
    Type:                  Failure

'''


Expected results:

The kubeletconfigs behavior should be consistent with the documentation. Either the docs should be updated to reflect the current behavior of the controller or the controller should be updated to match the described behavior in the docs. 


Additional info:


[1] - https://docs.openshift.com/container-platform/4.5/rest_api/machine_apis/kubeletconfig-machineconfiguration-openshift-io-v1.html
[2] - https://github.com/openshift/machine-config-operator/blob/master/pkg/controller/kubelet-config/kubelet_config_controller.go#L614-L615