[COOK-3528] Fix 'ufw::recipes' and 'ufw::databag' crash

Fixes https://tickets.opscode.com/browse/COOK-3528

If a node had no firewall rules already it would crash when trying to do
the concat operation.

There was already a PR (https://github.com/opscode-cookbooks/ufw/pull/7/files) for the same bug but it wasn't merged and was using 'set' too much in my opinion.