make it possible to kick buildbot without a no-op [testfix]

I'm going to tackle this for my first build engineer thing by getting the 'force build' button to appear.

If you enable the force build feature in the config, the button doesn't appear unless there is a build already running. I think this is because the a latent build slave is not connected, in buildbot jargon (though I think it is always online?).

If you then visit $builder_url/force in your browser, you manage to force a build, but then this happens:

Traceback (most recent call last):
  File "/usr/lib/python2.5/site-packages/buildbot/process/buildstep.py", line 686, in startStep
    d.addCallback(self._startStep_2)
  File "/usr/lib/python2.5/site-packages/twisted/internet/defer.py", line 191, in addCallback
    callbackKeywords=kw)
  File "/usr/lib/python2.5/site-packages/twisted/internet/defer.py", line 182, in addCallbacks
    self._runCallbacks()
  File "/usr/lib/python2.5/site-packages/twisted/internet/defer.py", line 317, in _runCallbacks
    self.result = callback(self.result, *args, **kw)
--- <exception caught here> ---
  File "/usr/lib/python2.5/site-packages/buildbot/process/buildstep.py", line 709, in _startStep_2
    skip = self.start()
  File "/usr/lib/python2.5/site-packages/buildbot/steps/source.py", line 184, in start
    self.startVC(branch, revision, patch)
  File "/usr/lib/python2.5/site-packages/buildbot/steps/source.py", line 829, in startVC
    self.args['repourl'] = self.baseURL + branch
exceptions.TypeError: cannot concatenate 'str' and 'NoneType' objects

Trying to bodge it by visiting a URL like "https://lpbuildbot.canonical.com/builders/db_lp/force?branch=bzr+ssh://bazaar.launchpad.net/~launchpad-pqm/launchpad/db-devel" doesn't seem to work.