fix(runtime-vapor): use slot fallback on initial render

[edison1105]

• Playground with vapor branch
• the slot fallback content does not show after initial render.

[netlify]

✅ Deploy Preview for vapor-repl ready!

Name	Link
🔨 Latest commit	53564e0
🔍 Latest deploy log	https://app.netlify.com/sites/vapor-repl/deploys/67ecf9c37cdc5300085f358b
😎 Deploy Preview	https://deploy-preview-13144--vapor-repl.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[pkg-pr-new]

Open in StackBlitz

@vue/compiler-core

npm i https://pkg.pr.new/@vue/compiler-core@13144

@vue/compiler-dom

npm i https://pkg.pr.new/@vue/compiler-dom@13144

@vue/compiler-ssr

npm i https://pkg.pr.new/@vue/compiler-ssr@13144

@vue/compiler-sfc

npm i https://pkg.pr.new/@vue/compiler-sfc@13144

@vue/compiler-vapor

npm i https://pkg.pr.new/@vue/compiler-vapor@13144

@vue/reactivity

npm i https://pkg.pr.new/@vue/reactivity@13144

@vue/runtime-core

npm i https://pkg.pr.new/@vue/runtime-core@13144

@vue/runtime-dom

npm i https://pkg.pr.new/@vue/runtime-dom@13144

@vue/runtime-vapor

npm i https://pkg.pr.new/@vue/runtime-vapor@13144

@vue/server-renderer

npm i https://pkg.pr.new/@vue/server-renderer@13144

@vue/shared

npm i https://pkg.pr.new/@vue/shared@13144

vue

npm i https://pkg.pr.new/vue@13144

@vue/compat

npm i https://pkg.pr.new/@vue/compat@13144

commit: 53564e0