Ensure the key is present in the dict before calling it
    
    Older project managed to get '' as milestone which has since been
    blocked and those milestones are cleaned, so just be sure it exists
    before calling it.
    
    Fixes https://pagure.io/pagure/issue/3913
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>