Minor improvement for array value linter rule

Summary:
This linter rule fails on multi-line arrays with no whitespace before the first array value. Specifically, the following exception is thrown:

Fatal error: Call to a member function isAnyWhitespace() on boolean in arcanist/src/lint/linter/xhpast/rules/ArcanistArrayValueXHPASTLinterRule.php on line 40

Test Plan: Added another test case.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D13856