Fix 404 for blurblogs if photo_url is None

Fixes the following:
[...]
<link rel="shortcut icon" href="None">
<link rel="icon" href="None">
[...]

-> Failed to load resource: the server responded with a status of 404 (NOT FOUND) http://menakite.newsblur.com/None

Note: not tested - I actually can't test it.