Adjust exclude rules filtering to avoid passing null to strlen() to fix PHP 8 deprecation notice

[ocean90]

Fixes #1349.

This fixes a deprecation notice in PHP 8:

PHP Deprecated: strlen(): Passing null to parameter #1 ($string) of type string is deprecated in classes/class-log.php on line 217

The value of my exclude rule was:

Array
(
    [connector] =>
    [context] =>
    [action] =>
    [ip_address] => 127.0.0.1
    [author] =>
    [role] =>
)

The fix is to check for null first.

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: Avoid deprecation notices in PHP 8.

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.