juju/juju — pull request #488 by axw (2014-08-11 09:46:37+00:00)
Title: state: don't issue $set/$unset with empty set

Mongo 2.6 complains if you attempt to perform
a $set or $unset operation with an empty set.
If the set is dynamic, we must be careful to
only issue the operation if non-empty.