Change ordergroup default in FinancialTransaction to NULL

The AllowEmtpyOrdergroupInFinancialTransaction migration only changed the
possibility to store NULL values, but did not set the default to NULL.

db/migrate/20181201000301_change_ordergroup_default_in_financial_transaction.rb

@@ -0,0 +1,13 @@
+class ChangeOrdergroupDefaultInFinancialTransaction < ActiveRecord::Migration
+  class FinancialTransaction < ActiveRecord::Base; end
+
+  def up
+    change_column_default :financial_transactions, :ordergroup_id, nil
+    FinancialTransaction.where(ordergroup_id: 0).update_all(ordergroup_id: nil)
+  end
+
+  def down
+    FinancialTransaction.where(ordergroup_id: nil).update_all(ordergroup_id: 0)
+    change_column_default :financial_transactions, :ordergroup_id, 0
+  end
+end