When returning false from requiresWorkspaceForPolling() method, an SCM may be presented 'null' for the
launcher, in addition to the workspace being null.
For the bitkeeper plugin, a launcher on the slave is necessary, but no existing workspace is necessary. To
make use of the master/slave arrangement, it seems difficult for any SCM to do polling without a launcher
available. Running everything from the master doesn't scale well.

I can copy all the logic from AbstractProject.pollSCMChanges to decide what to do when the launcher is
null, but that seems wasteful. It would be preferable to have another predicate that indicates a slave
launcher is necessary for polling.