Could you move this `if` to outside the `if-let`, as the false branch doesn't use `value`.

Also, I'd prefer not having to repeat the button lookup, i think it'd be neater if either:
* The "grab button out of form" logic was extracted from `form-with-submit` and shared with this
* Change `form-with-submit` to `form-and-submit` that returns a pair, and then `all-form-params` accepts the button element instead of re-doing the selector

I think I prefer the second option.