Add error message running "lipsum generate owners" with no repositories
Summary:
First check if a repository exists before trying to call `getPHID()` on `null`.
Closes T15083
Test Plan:
* Have some repositories; run `./bin/lipsum generate owners` and see it still works
* Probably have no repositories; run `./bin/lipsum generate owners` and see it fail (I did not test)
Reviewers: O1 Blessed Committers, avivey
Reviewed By: O1 Blessed Committers, avivey
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15083
Differential Revision: https://we.phorge.it/D26169