fix bug in namespace usage

[natefinch]

fixes-1461354

https://bugs.launchpad.net/juju-core/+bug/1461354
debug-log EOF: invalid tag and panic seen in state server logs

This was a result of me changing the namespace stored in the config to no longer store the leading "-". Instead, it just stores the namespace itself, and where we use it, we have to format it correctly. This seems like a much more sane way to do it, so the value you set is the value that exists, and we don't dictate formatting to everyone using the namespace.... however, it meant I forgot to check for an empty namespace in this one case.

(Review request: http://reviews.vapour.ws/r/1849/)

[natefinch]

Not sure what happened here, but re-PR'd in a different PR. Closing this one.