fix(ssr): hydrate strings in fragments

[jaulz]

Right now, it seems that the hydrateChildren function optimizes too much. When you have a custom component that uses renderSlot and that renders a simple string it fails:

This PR removes the current check optimized = optimized || !!parentVNode.dynamicChildren so strings are converted to VNodes before they are compared in hydrateNode:
https://github.com/vuejs/vue-next/blob/92f11d6740929f5b591740e30ae5fba50940ec82/packages/runtime-core/src/hydration.ts#L379-L430

[yyx990803]

Are you calling renderSlot manually? It's a compiler-only helper so the runtime will assume its return content is always normalized vnodes: https://template-explorer.vuejs.org/#eyJzcmMiOiI8Q29tcD5cblx0PHNsb3QgbmFtZT1cInRpdGxlXCI+ZmVzZjwvc2xvdD5cbjwvQ29tcD4iLCJvcHRpb25zIjp7ImhvaXN0U3RhdGljIjp0cnVlLCJjYWNoZUhhbmRsZXJzIjp0cnVlfX0=

You should not be using this in manual render function code.

[jaulz]

@yyx990803 thanks for your feedback and explains a lot! 😊 Maybe it's worth a renaming (e.g. with a prefix) to indicate internal usage only?