types/persist: omit Persist.AttestationKey based on IsZero (#18241)

IsZero is required by the interface, so we should use that before trying
to serialize the key.

Updates #35412

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

types/persist/persist.go

@@ -26,7 +26,7 @@ type Persist struct {
 	UserProfile       tailcfg.UserProfile
 	NetworkLockKey    key.NLPrivate
 	NodeID            tailcfg.StableNodeID
-	AttestationKey    key.HardwareAttestationKey `json:",omitempty"`
+	AttestationKey    key.HardwareAttestationKey `json:",omitzero"`
 
 	// DisallowedTKAStateIDs stores the tka.State.StateID values which
 	// this node will not operate network lock on. This is used to