Don't try to load user profile images in PhabricatorPeopleQuery if no users have any

Summary:
Fixes T3810. In PhabricatorPeopleQuery, we issue an unnecessary query like this:

  SELECT f.* FROM file f WHERE (f.phid IN ('')) ORDER BY f.id DESC

...if we're loading a user without a profile picture. Filter the file PHIDs before loading them to prevent this.

This doesn't change anything, but saves us a spurious/silly query.

Also makes `PhabricatorPeopleProfileController` use `needProfileImage()`, moving us closer to getting rid of `loadProfileImageURI()` eventually.

Test Plan: Looked at profiles of users with and without profile pictures. Checked query log in DarkConsole.

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T3810

Differential Revision: https://secure.phabricator.com/D6913