Fix a compatibility issue with ArcanistUnsafeDynamicStringXHPASTLinterRule

Summary: A user reported to me that `arc lint` was failing with an error message along the lines of "argument 1 passed to `idx` must be of type array, bool given". I suspect that the user is running an older version of PHP, which means that `array_combine(array(), array())` is returning `false` instead of the expected `array()`. Instead, avoid calling `array_combine` on an empty array.

Test Plan: I don't have PHP 5.3 easily accessible to test this.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D14567

src/lint/linter/xhpast/rules/ArcanistUnsafeDynamicStringXHPASTLinterRule.php

@@ -70,9 +70,11 @@ final class ArcanistUnsafeDynamicStringXHPASTLinterRule
     AASTNodeList $calls,
     array $safe) {
 
-    $safe = array_combine(
-      array_map('strtolower', array_keys($safe)),
-      $safe);
+    if ($safe) {
+      $safe = array_combine(
+        array_map('strtolower', array_keys($safe)),
+        $safe);
+    }
 
     foreach ($calls as $call) {
       $name = $call->getChildByIndex(0)->getConcreteString();