relation_get() on a non-set relation setting does not return None

[Adam Gandelman]

Not sure if this was ever the expected behavior or if something changed, but calling relation_get() on a non-set relation setting does catch the ValueError from json.loads() and does not return None. Instead, returns an empty string, unlike config() which always returns None for an unset config value:

(Pdb) print config('foobar')
None
(Pdb) print relation_get('foobar')

(Pdb)

Worth noting the inconsistency in output from the relation-get command, using python juju 0.7+bzr628+bzr631~precise1:

# config-get foobar --format=json
null
# relation-get foobar --format=json
""