fix helpers.columnate bug when list is empty

fix helpers.columnate bug when list is empty

When the list given to the columnate function is empty, the function
raises an exception when determining the max(len of all elements), since
the list given to max is empty.

One indirect example of when this bug is apparent is in the 'bup ftp'
command when listing an empty directory:

    bup> ls backupname/latest/etc/keys
    error: max() arg is an empty sequence

Add a special condition at the beginning of the columnate function that
returns an empty string if the list of elements is empty.

Signed-off-by: Gabriel Filion <lelutin@gmail.com>