Fix RuntimeException: min(): Array must contain at least one element

Summary:
Make sure that $epochs[] is never empty but contains 0 so min($epochs) will not complain.

Closes T15623

Test Plan: Carefully read the code (as I am unaware of reproduction steps).

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15623

Differential Revision: https://we.phorge.it/D25417