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