In the scenario where the opponent has no monster and you tributed both of yours, I would have to say that Torrential Tribute could not be activated, simply because, as you said, it would not do anything. Broken down further:
1) Summon Hand of Nephthys
2) As turn player, priority to activate the ignition effect, and tribute both monsters, no monsters on the field now.
3) The opponent now has an opportunity to respond, with the timing window for Torrential Tribute still perfect, but since Torrential Tribute won't do anything at this point, it would be illegal to activate.
4) Unable to respond, Sacred Phoenix of Nephthys comes out onto the field.
5) The opponent has an opportunity to now activate Torrential Tribute since it will be able to destroy something.