artist urls: make normalized_url nullable.

Make the normalized_url column nullable in preparation for dropping it.

db/migrate/20220403042706_make_normalized_url_nullable_on_artist_urls.rb

@@ -0,0 +1,5 @@
+class MakeNormalizedURLNullableOnArtistURLs < ActiveRecord::Migration[7.0]
+  def change
+    change_column_null :artist_urls, :normalized_url, true
+  end
+end