I think we have to see Michael's "nothing"-offer in connection to "All that's left is our friendship." These two statements are in fact contradictory. But only with the initial threatening, Michael and Tom can be sure that Geary got the message. Without the threat he might have believed that he really had killed her by himself.
Actually the statements are not contradictory at all. The offer of "nothing" comes after Geary makes it clear he doesn't like Michael, and Michael's kind with their oily hair and silk suits passing themselves off as real Americans. Michael, through Tom turns the tables on Geary 100% with the hooker. Tom leads him through a series of questions where it becomes clear that without Michael's help there is a damn good circumstantial case for manslaughter, against the senator and the likelihood that his career is over. In classic Corleone fashion, Tom explains the Senator has only one choice, and that is to accept Michael's "friendship," because that's literally all he has left if he wants to continue his career.
