Fix issue with checkbox custom controls that have no strings defined

Summary: Fixes T3864. If you define a "bool" control but don't define a
corresponding "strings", we currently fatal when trying to `idx()` into
`null`.

Auditors: btrahan

src/infrastructure/customfield/standard/PhabricatorStandardCustomField.php

@@ -9,7 +9,7 @@ abstract class PhabricatorStandardCustomField
   private $fieldDescription;
   private $fieldConfig;
   private $applicationField;
-  private $strings;
+  private $strings = array();
   private $caption;
   private $fieldError;
   private $required;