handle opening empty images.json while re-running pungi-koji in debug mode
    
    If there is an empty images.json file exists, re-running pungi-koji
    in debug-mode will fail with exception. We can just ignore the exception
    in such cases, pungi-koji will create a correct images.json later.
    
    Signed-off-by: Qixiang Wan <qwan@redhat.com>