The previous implementation tried to rename the tag named "default" to
the first tag in `cfg.tags`. This was a wrong approach because if a
tag with the same name already existed, the renaming failed and the
default tag would continue to exist.
The looking up of the default tag also contained a bug because it
should have used `by-name` in the path.
The default value for `xsession.windowManager.herbstluftwm.tags` is an
empty list, but the config file uses `builtins.head` on it, which causes
an error upon evaluation. With this change the tags configuration is
skipped if the list is empty.
