Filter null values in PRODUCT() function

[MarkBaker]

…lculation in any way, and actually require additional code handling if they're present within the calculation itself

This is:

- [X] a bugfix
- [ ] 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?

Issue #2833

[oleibman]

It all looks good to me.

[MarkBaker]

ty