tailcfg: add more IsZero methods

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

tailcfg/tailcfg.go

@@ -41,8 +41,16 @@ func (u NodeID) IsZero() bool {
 
 type GroupID ID
 
+func (u GroupID) IsZero() bool {
+	return u == 0
+}
+
 type RoleID ID
 
+func (u RoleID) IsZero() bool {
+	return u == 0
+}
+
 type CapabilityID ID
 
 // MachineKey is the curve25519 public key for a machine.