fix major bug when receiving empty ouput

sinker/sinker.go

@@ -103,19 +103,22 @@ func (s *SQLSinker) HandleBlockScopedData(ctx context.Context, data *pbsubstream
 
 	dbChanges := &pbdatabase.DatabaseChanges{}
 	mapOutput := output.GetMapOutput()
-	if !mapOutput.MessageIs(dbChanges) && mapOutput.TypeUrl != "type.googleapis.com/sf.substreams.database.v1.DatabaseChanges" {
-		return fmt.Errorf("mismatched message type: trying to unmarshal unknown type %q", mapOutput.MessageName())
-	}
 
-	// We do not use UnmarshalTo here because we need to parse an older proto type and
-	// UnmarshalTo enforces the type check. So we check manually the `TypeUrl` above and we use
-	// `Unmarshal` instead which only deals with the bytes value.
-	if err := proto.Unmarshal(mapOutput.Value, dbChanges); err != nil {
-		return fmt.Errorf("unmarshal database changes: %w", err)
-	}
+	if mapOutput.String() != "" {
+		if !mapOutput.MessageIs(dbChanges) && mapOutput.TypeUrl != "type.googleapis.com/sf.substreams.database.v1.DatabaseChanges" {
+			return fmt.Errorf("mismatched message type: trying to unmarshal unknown type %q", mapOutput.MessageName())
+		}
 
-	if err := s.applyDatabaseChanges(dbChanges, data.Clock.Number, data.FinalBlockHeight); err != nil {
-		return fmt.Errorf("apply database changes: %w", err)
+		// We do not use UnmarshalTo here because we need to parse an older proto type and
+		// UnmarshalTo enforces the type check. So we check manually the `TypeUrl` above and we use
+		// `Unmarshal` instead which only deals with the bytes value.
+		if err := proto.Unmarshal(mapOutput.Value, dbChanges); err != nil {
+			return fmt.Errorf("unmarshal database changes: %w", err)
+		}
+
+		if err := s.applyDatabaseChanges(dbChanges, data.Clock.Number, data.FinalBlockHeight); err != nil {
+			return fmt.Errorf("apply database changes: %w", err)
+		}
 	}
 
 	if data.Clock.Number%s.batchBlockModulo(data, isLive) == 0 {