artists: fix artist finder returning wrong results when given nil url.

Fix the artist finder returning incorrect results when given a nil URL.
This only happened when an artist with a URL like this existed:

    http:///blog.naver.com/dan_rak

Note the triple `///`; the extra `/` messed up the artist finder.

The artist finder may be given a nil URL when a source strategy returns
a nil profile URL, usually because the source is bad_id.

app/logical/artist_finder.rb

@@ -154,6 +154,8 @@ module ArtistFinder
   # @param url [String] the artist profile URL
   # @return [Array<Artist>] the list of matching artists
   def find_artists(url)
+    return Artist.none if url.blank?
+
     url = ArtistURL.normalize_normalized_url(url)
 
     # First try an exact match

app/logical/sources/strategies/base.rb

@@ -145,7 +145,7 @@ module Sources
       end
 
       def artists
-        ArtistFinder.find_artists(profile_url.to_s)
+        ArtistFinder.find_artists(profile_url)
       end
 
       # A new artist entry with suggested defaults for when the artist doesn't