fix(model): avoid throwing error when bulkSave() called on a document with no changes

Fix #10437

Author: vkarpov15

lib/model.js

@@ -3729,18 +3729,21 @@ Model.buildBulkWriteOperations = function buildBulkWriteOperations(documents, op
  }
 
  const delta = document.$__delta();
- const where = document.$__where(delta[0]);
- const changes = delta[1];
-
- _applyCustomWhere(document, where);
-
- document.$__version(where, delta);
 
  if (document.isNew) {
+ // new document
  accumulator.push({
  insertOne: { document }
  });
- } else if (!utils.isEmptyObject(changes)) {
+ } else if (delta != null && !utils.isEmptyObject(delta[0])) {
+ // document with changes
+ const where = document.$__where(delta[0]);
+ const changes = delta[1];
+
+ _applyCustomWhere(document, where);
+
+ document.$__version(where, delta);
+
  accumulator.push({
  updateOne: {
  filter: where,