Fix the validator for the 'description' attribute of Agents, allowing
it to be set to None.
Fix an API test that had two problems:
1. It was not restoring the description to the agent it had updated
2. It was retoring the description to '' instead of None.
Closes-Bug: #1466642
Change-Id: I50723e1346be0953d26216ba24907bac008ccfb6
(cherry picked from commit ee14186fbb)