bugfix: std::string() does not like NULL on construction
bugfix: std::string() does not like NULL on construction

not having DISPLAY set (eg: on a headless system) caused fluxbox to crash.

lukoko/fluxbox,Arkq/fluxbox,olivergondza/fluxbox,lukoko/fluxbox,naimdjon/fluxbox,luebking/fluxbox,naimdjon/fluxbox,Arkq/fluxbox,lukoko/fluxbox,Arkq/fluxbox,luebking/fluxbox,lukoko/fluxbox,olivergondza/fluxbox,luebking/fluxbox,luebking/fluxbox,Arkq/fluxbox,luebking/fluxbox,Arkq/fluxbox,olivergondza/fluxbox,naimdjon/fluxbox,olivergondza/fluxbox,olivergondza/fluxbox,naimdjon/fluxbox,naimdjon/fluxbox,lukoko/fluxbox