Avoid deprecation warning for missing page GET input

[ocean90]

Also move the is_admin() check to the top for improved readability.

Fixes #1439.

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 warnings in PHP 8 for missing page GET input.

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.

[kadamwhite]

I came here intending to open a similar PR, I can confirm that this patch fixes the warning we encountered.

[kasparsd]

This is great, thank you!