Ods Reader No Datatype for Null Value

[oleibman]

Issue #4435 was initially described incorrectly. While investigating the original description, I came upon this problem. Ods Reader is trying to set some cells to null without supplying a valid DataType to setValueExplicit, causing that method to throw an exception. Reader is changed to no longer call that method when value is null and DataType is 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

[oleibman]

No idea why Scrutinizer error-ed. Don't really care.