pkgset: Skip adding modulemd if there is none
    
    When libmodulemd is available, but the compose contains no modules, we
    don't want to add the modular metadata anywhere. This patch actually
    avoids a crash as `ModuleIndex.dump_to_string()` will return `None` if
    there is no data.
    
    JIRA: COMPOSE-3662
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>