Fix libphutil's readable JSON serializer for empty arrays
Summary: The range() test has a bad edge case for empty arrays and caused us to
encode them as '{}' instead of '[]'. Using '[]' is more consistent with PHP's
behavior.
Test Plan: Ran unit test.
Reviewers: btrahan, jungejason, nh, tuomaspelkonen, aran
Reviewed By: btrahan
CC: aran, btrahan
Differential Revision: 1119