core: Prevent critical if there's no User-Agent
core: Prevent critical if there's no User-Agent

jku/rygel,jku/rygel,Pelagicore/Media-Manager-Rygel-patched,GNOME/rygel,GNOME/rygel,GNOME/rygel,luisbg/rygel,Pelagicore/Media-Manager-Rygel-patched,Tarnyko/rygel,Pelagicore/Media-Manager-Rygel-patched,Tarnyko/rygel