gpgverify check should handle missing %prep
    
    Having a `%prep` section is not mandatory (it generates an rpmlint warning, not error), but not having it causes the gpgverify SHOULD plugin to fail as it assumes the section exists.
    
    Handle this by using `or []` before iterating over the section's lines,
    as `get_section` returns `None` on non-existent sections.
    
    Fixes #448.
    
    Signed-off-by: Michel Alexandre Salim <salimma@fedoraproject.org>