Fix inventory service to return 'nil' when a node is not found

The previous commit had a side effect of causing the facts
terminus' `find` method to return an empty array when the
requested node did not exist.  This commit changes the behavior
back to what it was previously--to return `nil` instead.
Merge pull request #419 from cprice-puppet/but/master/18934-inventory-service-should-return-nil-for-unknown-node

Fix inventory service to return 'nil' when a node is not found