challenger: skip trying to load+parse AMP invoices

AMP invoiecs don't have a payment hash, as it's based on payment addr
and set id.

challenger/lnd.go

@@ -102,6 +102,12 @@ func (l *LndChallenger) Start() error {
 	// updates for new invoices and/or newly settled invoices.
 	l.invoicesMtx.Lock()
 	for _, invoice := range invoiceResp.Invoices {
+		// Some invoices like AMP invoices may not have a payment hash
+		// populated.
+		if invoice.RHash == nil {
+			continue
+		}
+
 		if invoice.AddIndex > addIndex {
 			addIndex = invoice.AddIndex
 		}
@@ -207,6 +213,12 @@ func (l *LndChallenger) readInvoiceStream(
 		default:
 		}
 
+		// Some invoices like AMP invoices may not have a payment hash
+		// populated.
+		if invoice.RHash == nil {
+			continue
+		}
+
 		hash, err := lntypes.MakeHash(invoice.RHash)
 		if err != nil {
 			log.Errorf("Error parsing invoice hash: %v", err)