Suggest to return [] when no image in the pool

[<Eric_YH_Chen@xxxxxxxxxxx>]

Hi, developers,

When I used the API in rbd.py, 

I found RBD().list(ioctx) would return [""] when there is no image in the pool.

I suggest it should return [] in this case. It would avoid some programming problem. Thanks!

regards,

Eric/Pjack

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