Fix call to non-existing newWorkCommands() in NuanceItemType

Summary:
rPe1b8532e2478395bd1f36baecb998adeef5bea3d introduced an implementation of `newWorkCommands()` in the child class `NuanceFormItemType` which is called in parent `NuanceItemType::buildWorkCommands()` without being defined in that parent class.
Thus define `newWorkCommands()` as an abstract class in parent class `NuanceItemType` and add an implementation returning null in the other child class `NuanceGitHubEventItemType` to make static code analyzers happy.

Test Plan: Carefully read the code.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25655

src/applications/nuance/item/NuanceGitHubEventItemType.php

@@ -20,6 +20,10 @@ final class NuanceGitHubEventItemType
     return $this->newRawEvent($item)->getEventFullTitle();
   }
 
+  protected function newWorkCommands(NuanceItem $item) {
+    return array();
+  }
+
   public function canUpdateItems() {
     return true;
   }

src/applications/nuance/item/NuanceItemType.php

@@ -6,6 +6,8 @@ abstract class NuanceItemType
   private $viewer;
   private $controller;
 
+  abstract protected function newWorkCommands(NuanceItem $item);
+
   public function setViewer(PhabricatorUser $viewer) {
     $this->viewer = $viewer;
     return $this;