Ods Reader Allow Omission of Some Page Settings Tags

[oleibman]

Fix #4099. Ods Reader was expecting there to always be header-style and footer-style tags when page-layout tag is present, but these need not exist. It seemed like there might be other exposures along this line in readPageSettingStyles; rather than waiting for a problem report to show up for each, the code is updated to use ->item(0) in place of [0] when appropriate, and make use of the nullsafe ?-> operator introduced with Php8.

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.