multi: Send empty VSP policy fields

This changes the VSP library functions to fill in missing policy choices
with empty maps instead of nil maps.

This fixes an issue where certain VSPs will fail to accept the policy
calls because those fields are required by the API but not sent.

internal/rpc/jsonrpc/methods.go

@@ -4687,6 +4687,18 @@ func (s *Server) setVoteChoice(ctx context.Context, icmd any) (any, error) {
 func (s *Server) updateVSPVoteChoices(ctx context.Context, w *wallet.Wallet, ticketHash *chainhash.Hash,
 	choices map[string]string, tspendPolicy map[string]string, treasuryPolicy map[string]string) error {
 
+	// Ensure empty (as opposed to nil) choices on every category (required
+	// by the contract of the VSP API).
+	if choices == nil {
+		choices = map[string]string{}
+	}
+	if tspendPolicy == nil {
+		tspendPolicy = map[string]string{}
+	}
+	if treasuryPolicy == nil {
+		treasuryPolicy = map[string]string{}
+	}
+
 	if ticketHash != nil {
 		vspHost, err := w.VSPHostForTicket(ctx, ticketHash)
 		if err != nil {

wallet/wallet.go

@@ -408,7 +408,7 @@ func (w *Wallet) AgendaChoices(ctx context.Context, ticketHash *chainhash.Hash)
 	const op errors.Op = "wallet.AgendaChoices"
 	version, deployments := CurrentAgendas(w.chainParams)
 	if len(deployments) == 0 {
-		return nil, 0, nil
+		return map[string]string{}, 0, nil
 	}
 
 	choices = make(map[string]string, len(deployments))