Fix a bug in FlashMessages on Node 0.12
Fix a bug in FlashMessages on Node 0.12

Apparently, calling Object.keys on a non-object (in our case undefined)
fails on Node 0.12, which is why Travis was failing:
https://travis-ci.org/keystonejs/keystone/jobs/141745607

This commit fixes that by only using Object.keys if an object exists

creynders/keystone,creynders/keystone