Fix a panic in handling one-time auth tokens, used to support `nomad ui
--authenticate`.

If the nomad leader is a 1.1.x with some servers running as 1.0.x, the
pre-1.1.0 servers risk crashing and the cluster may lose quorum. That
can happen when `nomad authenticate -ui` command is issued, or when the
leader scans for expired tokens every 10 minutes.

Fixed #10943 .