Subversion: fix empty "Authored on" date on commit pages

Summary:
Subversion does not allow to have local commits. So, manage this case.

Closes T15612

Test Plan:
Surf a git commit: nothing changed.

Surf a svn commit: the "Authored on" date is not empty anymore.

Reviewers: O1 Blessed Committers, aklapper

Reviewed By: O1 Blessed Committers, aklapper

Subscribers: aklapper, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15612

Differential Revision: https://we.phorge.it/D25796