Ensure array key exists

The service entry point can be used outside a web context,
for example through CLI to run a MediaWiki maintenance script.

As such, we can't make assertions on keys available in the $_SERVER array.

Summary: Ref T1675

Test Plan: sudo -u mediawiki mw update agora

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1675

Differential Revision: https://devcentral.nasqueron.org/D2481