Check if OLD is set when migrating issue assignees

The procedure for migrating issue assignees is invoked on both an INSERT
and UPDATE. This means it will fail for INSERTs because OLD is set to
NULL in this case.

db/migrate/20170516153305_migrate_assignee_to_separate_table.rb

@@ -47,7 +47,7 @@ class MigrateAssigneeToSeparateTable < ActiveRecord::Migration
             RETURNS trigger AS
             $BODY$
             BEGIN
-              if OLD.assignee_id IS NOT NULL THEN
+              if OLD IS NOT NULL AND OLD.assignee_id IS NOT NULL THEN
                   DELETE FROM issue_assignees WHERE issue_id = OLD.id;
               END IF;