Low level force damage spells are a bit weird in what they can target, but I think it's pretty easy to justify. You can apply quite a bit of electricity to a piece of metal without harming it, more than enough to kill a person or at least cause significant burns. Enough energy though and that metal will melt.
Same with fire spells, unless the target is extremely flammable low level fire spells are just a quick blast. May leave a scorch mark but that's it. It's the difference between passing your finger through the flame of a candle and the flame of a cutting torch. The former doesn't hurt, I wouldn't try the latter unless you want a trip to the doctor.
As far as the OP, I'd probably have the spell go off but then indicate that they splash off the statue harmlessly. It is interesting that they changed their mind about spells with invalid targets costing a spell slot in Xanathar's, there was a podcast at one time that said that the intent was that they would not.
Also, I let players call for intitiative any time they want. In this case they were attacking the gazebo, I wouldn't want to disillusion them.