In the logic that was attempting to handle the case where the local repo doesn't
have any bookmarks, the assumption was being made that tip resolved to a
non-null revision. In the case of a totally empty local repo, however, that
isn't a valid assumption, and resulted in attempting to set the master ref
to None, which broke dulwich.
The "fix", which avoids the traceback and allows the push to complete (though
still do nothing, since in this case there aren't any changes to push), is to
not tweak the refs at all if tip is nullid. Leaving the special capabilities
ref and not adding a master ref appears to be fine in this case.