artists: fix validation error when url_string has leading spaces.

Bug: if the url_string had leading spaces, when it was split the
resulting array contained the empty string, which led to a validation
error when trying to save the urls.

Fixup for #3247.

app/models/artist.rb

@@ -67,7 +67,7 @@ class Artist < ApplicationRecord
     def url_string=(string)
       @url_string_was = url_string
 
-      self.urls = string.split(/[[:space:]]+/).uniq.map do |url|
+      self.urls = string.scan(/[^[:space:]]+/).uniq.map do |url|
         self.urls.find_or_initialize_by(url: url)
       end
     end