Don't create a fragment at all for DOM-less components

[Rich-Harris]

Not a frequent occurrence, but it does happen, e.g. the <MapMarker> component in https://v3.svelte.technology/tutorial/context-api

[EmilTholin]

I've looked into this a bit, and I wonder if it isn't nicer just to keep a DOM-less component fragment looking like this...

function create_fragment(ctx) {
  return { c: noop, l: noop, m: noop, p: noop, i: noop, o: noop, d: noop };
}

...rather than checking for the existence of $$.fragment throughout the runtime code, since it's such a special case.