Add an official way to check if a slot is empty

[dorian-marchal]

Basically, this is exact same request than vuejs/core#4733 (now closed) but with an additional argument:

All the proposed ways to check if a slot is empty I found don't work with this use case: Vue SFC Playground

In this repro, the component should render hasContent [x] because the slot is not empty, but in this case the vNode received by isVNodeEmpty looks like that (a comment node):

Oddly, this works as expected if I give a default value to tag, e.g. { tag = 'div' } = {}

Previous discussions:

• How can I check if a Slot content is provided? core#9393
• Request to add an official way to check if slot is empty core#4733
• Check if slot is empty (options API) core#7002