fix: do not overwrite current context

[carlossg]

with a null/undefined one

When having multiple configs in KUBECONFIG only one of them may have the current context set
but it will be overwritten by an empty value if preserveContext is not explicitly set
which is not the case when using kubectl binary

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[brendandburns]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: brendandburns, carlossg

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[brendandburns]

Any chance you can cherry-pick this onto the release-1.x branch also?

[carlossg]

sure, #1488