WASHINGTON, Feb 7 (Reuters) - U.S. President Joe Biden on Monday warned that if Russia invades Ukraine, there would be no Nord Stream 2, but did not specify how he would go about ensuring the controversial pipeline would not be used. Speaking at a joint news conference with German Chancellor Olaf Scholz, Biden said, "If Russia invades... again, then there will be longer Nord Stream 2. We will bring an end to it." When asked how he would do that, he responded, "I promise you we will be able to do it." Europe's most divisive energy project, Nord Stream 2...