由于签名缺失，这份文件被视为无效。

The document was considered ___null due to the missing signature.

他的努力似乎毫无意义，因为最终结果为零。

His efforts seemed ___futile, as the final result amounted to zero.

她的承诺听起来空洞无物，毫无诚意。

Her promises sounded ___null and lacked sincerity.