Fix postgresql.rollbacks metric and don't crash when tags are None.

postgresql.rollbacks metric was collecting the wrong data due to a typo.
chef-datadog 1.0.1 can create a tags entry with a None value, causing the
the agent to crash. This is now fixed.
Merge pull request #557 from DataDog/postgres-none-tags

Fix postgresql.rollbacks metric and don't crash when tags are None.