Fix ArcanistPHPCloseTagXHPASTLinterRule always bailing out
D13794 changed ArcanistPHPCloseTagXHPASTLinterRule to ignore inline HTML blocks, but selectDescendantsOfType returns an AASTNodeList (which always exists).
Instead, check that the count() of the node list is > 0.
empty.lint-test had to be changed, it wouldn't have been accepted had this rule not been broken before it was commited.
Added tests to cover ArcanistPHPCloseTagXHPASTLinterRule in the future.
Test Plan: arc unit
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Differential Revision: https://secure.phabricator.com/D18271