Don't expect module build tasks to have "rpms"
    
    There are module builds, which won't build any components.There are
    module builds, which won't build any components. These module builds
    won't have an "rpms" key in "tasks".
    
    Expect this to happen and don't fail when watching module builds of
    this kind.
    
    Merges: https://pagure.io/rpkg/pull-request/476
    
    Signed-off-by: Mariana Ulaieva <mulaieva@redhat.com>