Setting default minimumSeverity to zero from null

Summary:
In hphp's version of idx, it uses debug_rlog() which is not
available in phabricator's code. It will be executed if null is passed
as the second value to idx.

Test Plan:
ran arc lint which still report error message, and php
./scripts/arcanist.php unit src/lint/linter/xhpast/__test__ doesn't
throw.

Reviewed By: epriestley
Reviewers: tuomaspelkonen, epriestley
CC: aran, jungejason, epriestley
Differential Revision: 666

src/lint/engine/base/ArcanistLintEngine.php

@@ -65,7 +65,7 @@ abstract class ArcanistLintEngine {
   protected $charToLine = array();
   protected $lineToFirstChar = array();
   private $results = array();
-  private $minimumSeverity = null;
+  private $minimumSeverity = ArcanistLintSeverity::SEVERITY_DISABLED;
 
   private $changedLines = array();
   private $commitHookMode = false;