Adjust get_sources.sh to allow empty `sources` file
    
    There are situations in which one will want to use a `sources` file to
    indicate that they wish to use the flat dist-git layout.
    However, until now we did not allow empty `sources` file in flat dist-git
    layout.
    With this change we allow empty `sources` file and we will just echo
    something in the logs saying that this file is empty and bail.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>