Preventing duplicate empty string aliases in Phurl's

Summary: Ref T8992, Make it impossible to save an empty string alias for a Phurl.

Test Plan:
- Create two Phurl's with non-empty aliases
- Delete aliases for both Phurl's
- Previously, this wouldn't allow to save the second Phurl because of a duplicate alias. Current diff should save empty alias as `null`, not empty string.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Maniphest Tasks: T8992

Differential Revision: https://secure.phabricator.com/D14413

src/applications/phurl/editor/PhabricatorPhurlURLEditor.php

@@ -49,9 +49,13 @@ final class PhabricatorPhurlURLEditor
     switch ($xaction->getTransactionType()) {
       case PhabricatorPhurlURLTransaction::TYPE_NAME:
       case PhabricatorPhurlURLTransaction::TYPE_URL:
-      case PhabricatorPhurlURLTransaction::TYPE_ALIAS:
       case PhabricatorPhurlURLTransaction::TYPE_DESCRIPTION:
         return $xaction->getNewValue();
+      case PhabricatorPhurlURLTransaction::TYPE_ALIAS:
+        if (!strlen($xaction->getNewValue())) {
+          return null;
+        }
+        return $xaction->getNewValue();
     }
 
     return parent::getCustomTransactionNewValue($object, $xaction);