Fix: prevent php warnings

[Rushaway]

Description

Add check before using $_GET to prevent warnings

Motivation and Context

https://github.com/sbpp/sourcebans-pp/pull/821 and like page.banlist.php#L722

How Has This Been Tested?

Tested, no more warnings at all.

Screenshots (if appropriate):

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.