Iron Curtain can be applied to 0 units

[wippie-openra]

Especially in (laggy) team games this issue happened to me quite a bit. For example; mammoths move very slow. One might think its easy to apply Iron Curtain to slow moving units, but thats not the case.

I've seen lots of times where I organize my army. Put mammoths in front. Move command mammoths into base. A-move all others behind. Pull Iron Curtain at the last second. And miss.

I propose a dependence of at least 1 iron curtained unit or building to reset the timer and play the sound.

[PunkPun]

Same should be applied to chronosphere. Related to chrono #16425

[wippie-openra]

I didn’t include Chrono in the opening post, because Chrono is also used on enemy units. But I think the same principle should apply indeed.