[management] fix handling of empty network map during decode and encode (#6987)

shared/management/networkmap/decode.go

@@ -228,15 +228,17 @@ func DecodeEnvelope(env *proto.NetworkMapEnvelope) (*types.NetworkMapComponents,
 	return c, nil
 }
 
+// decodeAccountNetwork never returns nil — Calculate() dereferences
+// c.Network unconditionally, and servers that predate the fix omit the field
+// entirely from the empty-components envelope.
 func decodeAccountNetwork(an *proto.AccountNetwork) *types.Network {
+	n := &types.Network{}
 	if an == nil {
-		return nil
-	}
-	n := &types.Network{
-		Identifier: an.Identifier,
-		Dns:        an.Dns,
-		Serial:     an.Serial,
+		return n
 	}
+	n.Identifier = an.Identifier
+	n.Dns = an.Dns
+	n.Serial = an.Serial
 	if an.NetCidr != "" {
 		if _, ipnet, err := net.ParseCIDR(an.NetCidr); err == nil && ipnet != nil {
 			n.Net = *ipnet