add documentation for @{empty} variable to user guide
add documentation for @{empty} variable to user guide

waldenner/robotframework,fiuba08/robotframework,ldtri0209/robotframework,ldtri0209/robotframework,ldtri0209/robotframework,fiuba08/robotframework,fiuba08/robotframework,waldenner/robotframework,waldenner/robotframework,fiuba08/robotframework,ldtri0209/robotframework,waldenner/robotframework,waldenner/robotframework,fiuba08/robotframework,ldtri0209/robotframework