lipsum - tighten up some test data generation

Summary:
maniphest tasks were fataling with priority 0 before making sure to add the return null if new object trick to the maniphest pro editor.

pholio had a problem where if you had no jpegs you were walking off array_rand. tighten the math and then just return a built-in if no uploaded user images could be found. Fixes T3889.

Test Plan: bin/lipsum generate for a few minutes and no errors

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Maniphest Tasks: T3889

Differential Revision: https://secure.phabricator.com/D7222