setup_ui=False breaks

When specifying setup_ui=False to breezy.initialize(), you end up with ui set to None, which then leads to a method on None being called.

...
  File "/home/janitor/debian-janitor/janitor/trace.py", line 60, in <module>
    breezy.initialize(setup_ui=False)
  File "/home/janitor/debian-janitor/breezy/breezy/__init__.py", line 222, in initialize
    state._start()
  File "/home/janitor/debian-janitor/breezy/breezy/library_state.py", line 102, in _start
    self._ui.__enter__()
AttributeError: 'NoneType' object has no attribute '__enter__'