RollbackAction: Allow 'from' to be an empty string

RollbackAction: Allow 'from' to be an empty string

Fix regression from 9af38c046c86, which made 'from' a required
non-empty parameter where previously an empty value was allowed.

The rollback links always include a 'from' parameter, but it is
set to an empty string by Revision::getUserText if the current
revision has its username hidden.

Test plan:
* Go to action=history, tick latest revision and "Change visibility".
* Tick "Editor's username" and apply the change.
* Hit "rollback" on the history page.
* Before: "missing parameter" error.
  After: Success.

Bug: T141985
Change-Id: I20d23e2aeec858f82231910c030c14ffa3af656f