Fix highlighted code in Diffusion

Summary:
Ref T3141. This is a hacky fix until we can get something a little more sensible in place.

Currently, we sometimes end up with an empty text list, which causes file content to not display. Instead, generate the text list from the corpus if it isn't available.

Test Plan: Looked at a previously-broken source listing, saw code.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T3141

Differential Revision: https://secure.phabricator.com/D5871