What should I do? I left out the corn syrup

I imagine the corn syrup helps prevent the formation of sugar crystals during the heating phase. Seven minute frosting is just a swiss meringue of sorts in disguise, and that's what corn syrup does in that instance. Since you successfully made your frosting, it doesn't sound as if you have a problem that needs solving. I'd just serve it up and keep quiet, because I suspect the lucky recipients will eat it without a second thought.