Don't export keys for non-associative arrays

Summary: `phutil_var_export` can produce cleaner output if keys are omitted for non-associative arrays. Depends on D9935.

Test Plan: `arc unit`

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9936