R1:e2c0df4fb49f

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

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