Workaround for https://github.com/aws/aws-cli/issues/4843

aws.kubectl.kube.config

The problem is that if you have only one definition of a cluster, context or user in the `~/.kube/config` file 
and you delete one of them then `kubectl` will set deleted type to `null` instead of `[]`.

Then if you try to `update-kubeconfig` using aws cli command you will see an error:

* if `contexts: null`: Tried to insert into contexts,which is a <class 'NoneType'> not a <class 'list'>
* if `users: null`: Tried to insert into users,which is a <class 'NoneType'> not a <class 'list'>
* if `clusters: null`: 'NoneType' object is not iterable

Simple workaround for above cases is to replace all `null` with `[]` then run aws cli `update-kubeconfig` command, like this:

sed -i 's/: null/: []/g' ~/.kube/config && \
aws eks --region ${AWS_REGION} update-kubeconfig --name ${EKS_CLUSTER_NAME}