fix unmarshal err for blank field

[stewart-yu]

What this PR does / why we need it:

fix unmarshal err for blank field

E1021 16:37:15.521443       1 control_plane_init_mutex.go:67] init-locker "msg"="Failed to get information about the existing lock" "error"="failed to unmarshal semaphore information: unexpected end of JSON input" "cluster-name"="capi-quickstart" "configmap-name"="capi-quickstart-lock" "namespace"="default"

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

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[detiber]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: detiber, stewart-yu

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