multi: Send empty VSP policy fields

[matheusd]

This changes the call sites of 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.

Fix #2402

[jholdstock]

Maybe set these fields in updateVSPVoteChoices so it only has to exist in one place?

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 {

	if choices == nil {
		choices = map[string]string{}
	}
	if tspendPolicy == nil {
		tspendPolicy = map[string]string{}
	}
	if treasuryPolicy == nil {
		treasuryPolicy = map[string]string{}
	}

[matheusd]

Updated.

[jholdstock]

I figured this should live in the client rather than consumers having to deal with it decred/vspd#491