Fix ret_status for force_path=''
Fix ret_status for force_path=''

d54224d3c977850f5a1424e3b544786d6b11f70f and friends led to
instance.join passing an empty string to `force_path`, which makes the
join redirect fail.

DanielNeugebauer/adhocracy,DanielNeugebauer/adhocracy,alkadis/vcv,phihag/adhocracy,DanielNeugebauer/adhocracy,alkadis/vcv,liqd/adhocracy,alkadis/vcv,phihag/adhocracy,phihag/adhocracy,alkadis/vcv,phihag/adhocracy,DanielNeugebauer/adhocracy,liqd/adhocracy,phihag/adhocracy,DanielNeugebauer/adhocracy,liqd/adhocracy,liqd/adhocracy,alkadis/vcv