Does something like this look OK?

if (strlen($author_src)) {
  $author_style = ...;
  $author_icon = null;
} else {
  $author_style = null;
  $author_icon = id(new PHUIIconView())->setIcon('fa-something-something');
}

Then $author_link = javelin_tag(..., ..., $author_icon) down below. Basically, plug in a little identity icon instead of an actual background image if we have an Identity instead of a real user. This might make the CSS go craaaaaazy, though.

Alternatively, we could change if (!$author_phid) to if (!$author_phid || !$image_uri), which would "fix" this without a regression, but it would be nice to actually render something useful here instead of just giving up.

Failing this, you can use PhabricatorFile::loadBuiltin(...) and drop some static image file in resources/builtin/ to get a real image file, something like:

$identity_file = PhabricatorFile::loadBuiltin($viewer, 'identity-default-profile.png');
$identity_src = $identity_file->getViewURI();

We could probably drop this bit of code now -- it should be exceptionally rare now, since we'll almost always have an Identity even if we don't have a real user. (Before, it was a reasonable fallback if you committed with a weird name but we found a revision and could kind of guess things.)

I was leaning towards a change in PhabricatorRepositoryIdentityPHIDType to solve this everywhere we might want to render identity objects. Let me know if this is a Bad Idea for some reason.