message: don't break when adding a missing document
diff: @@ -6,10 +6,10 @@ class BulkOps {
  this.batchSize = batchSize || 1000;
  }
  - async add(operation, source) {
  + async add(operation, ...source) {
  this.queue.push(operation);
  - if (source) {
  - this.queue.push(source);
  + if (source.length > 0) {
  + this.queue.push(source[0] || {});
  }
  if (this.queue.length > this.batchSize) {
  await this.flush();
  