Refine handling of "@task" attributes in Diviner

Summary: Ref T13505. See that task for details. When a class has exactly one "@task" block, this API returns a string. Some day, this should be made more consistent.

Test Plan: Viewed a class with exactly one "@task", no more fatal. Viewed classes with zero and more than one "@task" attributes, got clean renderings.

Maniphest Tasks: T13505

Differential Revision: https://secure.phabricator.com/D21062

src/applications/diviner/controller/DivinerAtomController.php

@@ -435,6 +435,15 @@ final class DivinerAtomController extends DivinerController {
     $task_specs = array();
 
     $tasks = $symbol->getAtom()->getDocblockMetaValue('task');
+
+    if (!is_array($tasks)) {
+      if (strlen($tasks)) {
+        $tasks = array($tasks);
+      } else {
+        $tasks = array();
+      }
+    }
+
     if ($tasks) {
       foreach ($tasks as $task) {
         list($name, $title) = explode(' ', $task, 2);