Return empty array as a default value for repository symbol languages

Summary: Fixes T8283. The code in FileController assumes this value is an array.

Test Plan: Loaded any file in Diffusion in unconfigured repository, no more warning.

Reviewers: avivey, joshuaspence, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8283

Differential Revision: https://secure.phabricator.com/D12964