Remove not-null constraint on lock_version column if it exists

Closes #21678

db/migrate/20160830232601_change_lock_version_not_null.rb

+class ChangeLockVersionNotNull < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+    change_column_null :issues, :lock_version, true
+    change_column_null :merge_requests, :lock_version, true
+  end
+
+  def down
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20160827011312) do
+ActiveRecord::Schema.define(version: 20160830232601) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"