Fix missing WHERE clause in 20171106135924_issues_milestone_id_foreign_key migration

If milestone_id is NULL, we shouldn't need to update it to be NULL again. This
was causing us to touch almost all rows in the issues table for no good
reason.

Closes https://gitlab.com/gitlab-com/infrastructure/issues/3416

db/migrate/20171106135924_issues_milestone_id_foreign_key.rb

@@ -16,6 +16,7 @@ class IssuesMilestoneIdForeignKey < ActiveRecord::Migration
 
     def self.with_orphaned_milestones
       where('NOT EXISTS (SELECT true FROM milestones WHERE milestones.id = issues.milestone_id)')
+        .where('milestone_id IS NOT NULL')
     end
   end