Fix #3998: Some artist histories throw exceptions.

db/migrate/20181202172145_set_columns_to_not_null.rb

@@ -0,0 +1,17 @@
+class SetColumnsToNotNull < ActiveRecord::Migration[5.2]
+  def change
+    Artist.without_timeout do
+      change_column_null :artist_versions, :urls, false, '{}'
+      change_column_null :artist_versions, :other_names, false, '{}'
+      change_column_null :artist_versions, :group_name, false, ''
+      change_column_default :artist_versions, :group_name, from: nil, to: ""
+
+      change_column_null :artists, :other_names, false, '{}'
+      change_column_null :artists, :group_name, false, ''
+      change_column_default :artists, :group_name, from: nil, to: ""
+
+      change_column_null :wiki_pages, :other_names, false, '{}'
+      change_column_null :wiki_page_versions, :other_names, false, '{}'
+    end
+  end
+end