uploads: fix page_url for null strategy.

Fix the null source strategy setting the page URL. The page URL is
expected to be nil when we can't determine the page containing the image URL.

Fixes the upload_media_assets.page_url field being filled for uploads
from unknown sites.

app/logical/sources/strategies/null.rb

@@ -8,7 +8,7 @@ module Sources
       end
 
       def page_url
-        url
+        nil
       end
 
       def canonical_url