Fix mishandling of chunk threshold in Diffusion for installs with no chunk engines available

Summary: Fixes T10273. The threshold is null if no chunk engines are available, but the code didn't handle this properly.

Test Plan: Disabled all chunk engines, reloaded, hit issue described in task. Applied patch, got clean file content.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10273

Differential Revision: https://secure.phabricator.com/D15179