Make :data() on an empty Tensor return nil rather than segfault.

Fixes this:

    $ torch -e "torch.Tensor():data()"
    Torch 7.0  Copyright (C) 2001-2011 Idiap, NEC Labs, NYU
    Segmentation fault: 11

To get this instead:

    $ torch -e "print(torch.Tensor():data())"
    Torch 7.0  Copyright (C) 2001-2011 Idiap, NEC Labs, NYU
    nil

Also means this doesn't segfault: `torch.Tensor():apply(function() end)`