twitter: fix parsing of the artist name from the url.

Fixes URLs like https://twitter.com/intent/user?user_id=123 being
incorrectly normalized to http://twitter.com/intent/ in artist entries.

Also fixes the artist name to be taken from the url when it can't be
obtained from the api (when the tweet is deleted).

app/logical/sources/strategies/base.rb

@@ -132,7 +132,7 @@ module Sources
       # The url to use for artist finding purposes. This will be stored in the
       # artist entry. Normally this will be the profile url.
       def normalize_for_artist_finder
-        profile_url || url
+        profile_url.presence || url
       end
 
       # A unique identifier for the artist. This is used for artist creation.