Don't pass null value to rtrim()

Summary:
Fixes the following issue:
[29-Nov-2024 14:49:30 UTC] PHP Deprecated:
rtrim(): Passing null to parameter #1 ($string) of type string is deprecated
in /var/51-wwwroot/tools/gadgets/motd-variations.php on line 22

Test Plan: Browse /gadgets/motd-variations

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3612