fix MountFS.getsyspath for cases for nothing is mounted at that path
fix MountFS.getsyspath for cases for nothing is mounted at that path

PyFilesystem/pyfilesystem,flying-circus/pyfilesystem,pmitros/pyfs,Konubinix/pyfilesystem,pombreda/pyfilesystem,rivenz/pyfilesystem,piranna/pyfilesystem,zopyx/pyfilesystem,pmitros/pyfs