Add iterateOnlyExistingCells to Constructors

[oleibman]

Fix #3721. That issue can already be handled, but requires several statements when one ought to suffice. Adding an extra parameter to the RowCellIterator and ColumnCellIterator constructors is useful, easy, and becomes especially practical now that our supported Php releases all support named parameters (see new test Issue3721Test).

This is:

• 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 and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

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