Fix linking to issues or PRs when pre-viewing a comment
    
    Basically the old mechanism was retrieving an user '' which was not
    considered to be None, so when searching for the matched issue/PR,
    none were found and thus the text version was returned.
    With this change, we ask that username, project name or project
    namespace are all either something or None. The code is then
    able to find the issue/PR referenced in a comment when previewing
    it.
    
    Fixes https://pagure.io/pagure/issue/4319
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>