DarkConsole: Fix "strlen(null)" but for the "tab" query parameter

Summary:
See rPf4d2811

Bug: T15344

Test Plan: Opened DarkConsole with /~/, saw no errors in DarkConsole

Reviewers: O1 Blessed Committers, aklapper

Reviewed By: O1 Blessed Committers, aklapper

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25834