The arches aren't empty for spoiler-hiding, those scenes probably weren't fully polished. I mean, look at them in the trailer, they look pretty bad (Beaker looks so small compared to the other characters). They'll be touched up on before November.
I actually think it's both! I agree that looks like an "unfinished" shot in regards to postproduction (notice how pixelly it seems compared to just about every other shot in the trailer) and that it will be touched up, but i also think there's something very specific that they're (wisely) not spoilering. I actually was kind of sad they even showed the arches shot in the trailer as that would have been such a great moment to save as a surprise...but there's another gag in regards to that scene that you'll (hopefully) have to wait to see in the theatre.
BTW- i did see the comments on this page but i still have yet to read the entire thread but i'm going to make a safe prediction before doing so that within the 20 pages there's probably a very lively and long debate about the Fart Shoes...
My take is i have no problem with the joke itself - whoopie cushions are Very Fozzie...but even though you normally wouldn't actually hear him use the term "fart", in this context it worked because of Kermit's set-up and the wording of the punchline actually works better than "whoopie shoes", "noise shoes" etc...BUT what i DID object to was i really hated to see that moment used in the trailer - mainly because so many family-friendly films and/or franchise-based films often are made up of cheap tasteless body humor jokes (sometimes that ends up being the bulk of their content) that showing that moment in the trailer sent an unintended message to the viewer that "The Muppets" would fall in that same category of movie - which is really one of the worst things they could have done in regards to building up peoples' expectations.