If the k8s version argument passed to "upgrade plan" is missing
the logic should perform the following actions:
- fetch a "stable" version from the internet.
- if that fails, fallback to the local client version.

Currentely the logic fails because the cfg.KubernetesVersion is
defaulted to the version of the existing cluster, which
then causes an early exit without any ugprade suggestions.

See app/cmd/upgrade/common.go::enforceRequirements():
  configutil.FetchInitConfigurationFromCluster(..)

Fix that by passing the explicit user value that can also be "".
This will then make the "offline getter" treat it as an explicit
desired upgrade target.

In the future it might be best to invert this logic:
- if no user k8s version argument is passed - default to the kubeadm
version.
- if labels are passed (e.g. "stable"), fetch a version from the
internet.