Fix call to isGlobalDragAndDropUploadEnabled() from pages with no controller

Summary: Fixes T7902. Some kinds of error pages, including the 404 page, may not have a controller when they reach this method.

Test Plan: Hit 404 page.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7902

Differential Revision: https://secure.phabricator.com/D12538