Upstart scripts are being incorrectly identified as SysV init scripts
due to a logic error in the `service` module.
Because upstart uses multiple commands (`/sbin/start`, `/sbin/stop`,
etc.) for managing service state, the codepath for upstart sets
`self.svc_cmd` to an empty string on line 451.
Empty strings are considered a non-truthy value in Python, so
conditionals which are checking the state of `self.svc_cmd` should
explicitly compare it to `None` to avoid overlooking the fact that
the service may be controlled by an upstart script.
15235a718d