Excel Omits Between Operator for Data Validation

[oleibman]

Fix #3863. Data Validation default operator is between. When Excel writes out a data validation item, it may omit the operator. Xlsx reader will therefore initialize operator to null string. Issue indicates that user wants between returned for getOperator. A more serious problem is that isValid method does not handle this situation correctly. Data Validation is changed to set Operator to the default value if an attempt is made to set it to null string.

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?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
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