Minor, improve notification resiliance

Summary: The token transactions can publish empty transaction feed stories.
Stop them from doing that, and make notifications fail more quietly.

Auditors: btrahan

src/applications/notification/builder/PhabricatorNotificationBuilder.php

@@ -129,7 +129,12 @@ final class PhabricatorNotificationBuilder {
     $null_view = new AphrontNullView();
 
     foreach ($stories as $story) {
-      $view = $story->renderView();
+      try {
+        $view = $story->renderView();
+      } catch (Exception $ex) {
+        // TODO: Render a nice debuggable notice instead?
+        continue;
+      }
       $null_view->appendChild($view->renderNotification($this->user));
     }
 

src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php

@@ -2022,6 +2022,11 @@ abstract class PhabricatorApplicationTransactionEditor
     array $mailed_phids) {
 
     $xactions = mfilter($xactions, 'shouldHideForFeed', true);
+
+    if (!$xactions) {
+      return;
+    }
+
     $related_phids = $this->getFeedRelatedPHIDs($object, $xactions);
     $subscribed_phids = $this->getFeedNotifyPHIDs($object, $xactions);