R1:8b7a4670f8ea

Fix an issue where PhabricatorHandleList could fail to completely iterate

Fixes T8140. If a HandleList contained a null (because some caller sloppily added `null` as a handle), iteration (e.g., via `iterator_to_array()`) would abort prematurely.

In T8140, some of the project transactions add a `null` for an old file PHID when there's no old profile image.

Auditors: btrahan