Uploading changes to adapt code to PHP 8.1

[davefx]

Fixes #1349 .

I've fixed all the deprecation warnings I've found when running my site against PHP 8.1.

Checklist

• Project documentation has been updated to reflect the changes in this pull request, if applicable.
• I have tested the changes in the local development environment (see contributing.md).
• I have added phpunit tests.

Release Changelog

• Fix: PHP 8.1: PHP Deprecated: strlen(): Passing null to parameter #1 ($string) of type string is deprecated #1349, and other PHP-8.1 deprecation warnings

Release Checklist

• This pull request is to the master branch.
• Release version follows semantic versioning. Does it include breaking changes?
• Update changelog in readme.txt.
• Bump version in stream.php.
• Bump Stable tag in readme.txt.
• Bump version in classes/class-plugin.php.
• Draft a release on GitHub.

Change [ ] to [x] to mark the items as done.

[simonhammes]

@davefx Can you fix the conflicts in this PR?

[davefx]

@simonhammes Conflicts resolved.
Thanks!

[simonhammes]

@davefx Thank you very much. There's another deprecation notice that is being triggered on the main "Stream Records" screen:

timezone_name_from_abbr(): Passing null to parameter #1 ($abbr) of type string is deprecated (in classes/class-date-interval.php:46)

I think null should be replaced with an empty string in both timezone_name_from_abbr() calls in this file.

[delawski]

This will be superseded by #1494.

[delawski]

The PHP 8 compatibility issues have been addressed in 4.0.1 so I'm closing this PR.