If I use Castform's "Hurricane" attack and I have no benched Pokemon, do I have to discard an Energy card?
No, if you have no Benched Pokemon all Energy remains attached to Castform.

Other known variants

x2
-30
Wing Attack

HurricaneUnless this attack Knocks Out the Defending Pokémon, return the Defending Pokémon and all cards attached to it to your opponent's hand.
When hunting, it skims the surface of water at high speed to pick off unwary prey such as Magikarp.
If I use Castform's "Hurricane" attack and I have no benched Pokemon, do I have to discard an Energy card?
No, if you have no Benched Pokemon all Energy remains attached to Castform.
If I have only Special Energies attached (no Basic Energy) and use Tornadus' "Hurricane" attack, do I have to discard an energy or what?
If you don't have any Basic Energy cards attached to Tornadus, you simply ignore that part of the Hurricane attack.
If I have no Benched Pokemon and attack with Tornadus' "Hurricane", do I have to discard an energy?
No. If you have no Benched Pokemon there is no place to move an energy card, so you would not do that part of the attack.