hscontrol: preserve nil expiry on tailscaled restart

The guard added for #2862 in handleRegister checked
node.Expiry().Valid() before preserving node state on
Auth=nil + Expiry=zero registration requests. Valid() returns false
when node.Expiry is nil, the default for tagged nodes and for untagged
nodes registered against a preauth key with no default node.expiry
configured. Both fell through to handleLogout, which wrote
&time.Time{} (0001-01-01T00:00:00Z) over the original nil — the
user-visible 0001-01-01 expiry that `headscale nodes list` reports
after restart.

IsExpired() already returns false for both nil and zero-time, so the
Valid() check was redundant. Drop it so all nil-expiry nodes are
covered by the same preservation path.

Fixes #3170
Fixes #3262