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