Why is NoExt inhabited?

Hi Shayan, During code review a contributor asked about a TTG extension point. While answering his question I realized I didn't have a good explanation for why NoExt is inhabited; as far as I can tell it should rather be unhabited. If there is a reason for this then can you: 1. describe why this is so in #16863 [1] 2. open an MR documenting the reason in Note [Trees that Grow]. If not, perhaps we should consider changing it; the current state of things is a bit awkward. Cheers, - Ben [1] https://gitlab.haskell.org/ghc/ghc/issues/16863