Resource.update_attrs should check emptiness

[Brian Curtin]

The Resource.update_attrs method can sometimes be called with None, which causes issues when we try to iterate over the args and kwargs (which are None), causing a TypeError. This is currently worked around in https://review.openstack.org/#/c/193333 but we should make this method check the arguments before trying to iterate over them.

[Brian Curtin (brian.curtin)]

While this is still valid, I haven't seen this since reporting it and it's not a 1.0 blocker.