doc: prevent displaying empty version picker

PR-URL: https://github.com/nodejs/node/pull/15420
Fixes: https://github.com/nodejs/node/issues/15396


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>