state: preserve nil expiry on user owned registration when no default is configured

When a user owned node registers or re registers with a PreAuthKey and the
client sends zero client expiry while node.expiry is set to 0, the expiry
column ends up stored as 0001-01-01 00:00:00 instead of NULL. Two sites in
HandleNodeFromPreAuthKey build a non nil pointer to regReq.Expiry even when
the value is zero time, and the needsDefaultExpiry guard only replaces it
when s.cfg.Node.Expiry > 0, so the pointer to zero time survives to the
database.

Convert an unset regReq.Expiry to nil before handing it off so the
needsDefaultExpiry path is the only place that assigns a non nil pointer.

This is a narrower sibling of #3170 on the user owned PreAuthKey path. The
regression was introduced alongside the fix for #3111 in 6337a3db.