Allow empty array for `csprintf` when using "%Ls"

See:

  - <https://github.com/facebook/libphutil/issues/31>
  - <https://github.com/facebook/libphutil/pull/32>

Reviewed by: epriestley
Allow `PhutilTypeSpec` to work with classes and subclasses.

Summary: Fixes T4286.

Test Plan: `arc unit -- src/parser/`

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4286

Differential Revision: https://secure.phabricator.com/D7902