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