vault: protect against empty Vault secret response
vault: protect against empty Vault secret response

Also, fix a case where a successful second attempt of loading token can
cause a panic.

hashicorp/nomad,dvusboy/nomad,burdandrei/nomad,hashicorp/nomad,burdandrei/nomad,burdandrei/nomad,dvusboy/nomad,hashicorp/nomad,dvusboy/nomad,dvusboy/nomad,burdandrei/nomad,burdandrei/nomad,hashicorp/nomad,dvusboy/nomad,dvusboy/nomad,burdandrei/nomad,dvusboy/nomad,hashicorp/nomad,hashicorp/nomad,burdandrei/nomad