Slight Increase in Coverage Reading BIFF8

[oleibman]

After breaking up Xls Reader (PR #4118), it is a little easier to identify uncovered code. BIFF8 had no tests involving constant arrays. This PR adds some. Most of the work is in the tests, but some source code is modernized to use things like null coercion.

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).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.