Fix setting up the pagure hooks when there is no DOCS_FOLDER set
    
    On a pagure instance where the documentation hosting is disabled, the
    DOCS_FOLDER variable is set to ``None`` and this makes installing the
    pagure hook crash if we're not careful.
    
    This commit fixes this situation and test it so it doesn't happen again.
    
    Relates to https://pagure.io/pagure/issue/2462
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>