Provide an option for Cell Iterators to return a null or create a new cell when cell doesn't exist

[MarkBaker]

This is:

- [ ] a bugfix
- [X] a new feature
- [ ] refactoring
- [ ] additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

Provide an option that determines whether Cell Iterators should create a new cell or return a null when they try to access a cell that does not exist