Fix exception trying to rename user to their previous username

Summary:
When trying to rename a user, properly handle when the new username is the old username to avoid an exception.

EXCEPTION: (PhabricatorApplicationTransactionNoEffectException) Transactions have no effect:
  - Transaction (of type "user.rename") has no effect. at [<phorge>/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php:2823]

Closes T15795

Test Plan:

• As an admin, go to /people/manage/123/ and select Change Username. Use the old username as the new username and select Rename User. Without the patch, get an exception. With the patch, get a proper error message within the dialog, as if you tried to set the new username to an empty string.
• As an admin, go to /people/create/, select Create Standard User, and successfully create a new user account.
• In an anonymous browser window, go to /auth/start/, click Register New Account, and successfully create a new user account.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15795

Differential Revision: https://we.phorge.it/D25601