SimObject: make get_config_as_dict() tolerate undefined params
SimObject: make get_config_as_dict() tolerate undefined params

Without this patch, undefined params cause a cryptic KeyError
in multidict inside get_config_as_dict().  This patch lets
undefined params through get_config_as_dict() so they can
once again generate meaningful error messages later on in
the configuration process.

haowu4682/gem5,LingxiaoJIA/gem5,haowu4682/gem5,haowu4682/gem5,haowu4682/gem5,haowu4682/gem5,haowu4682/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,haowu4682/gem5,haowu4682/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,haowu4682/gem5