fix(runtime-core): delete stale slots which are present but undefined

[KaelWD]

fixes #9109

If slots.loader becomes undefined, slots.default in LoaderSlot was still the old value because the key is still present

<v-field>
  <template v-if="custom" #loader>
    <v-progress-linear />
  </template>
</v-field>

<LoaderSlot v-slots={{ default: slots.loader }} />

[haiweilian]

@sxzz Will it be merged？

[zhangzhonghe]

@KaelWD Thanks for the PR, could you add a test?

[KaelWD]

@zhangzhonghe tests updated

[sxzz]

/ecosystem-ci run

[vue-bot]

📝 Ran ecosystem CI: Open

suite	result
nuxt	✅ success
pinia	✅ success
quasar	✅ success
router	✅ success
test-utils	✅ success
vant	✅ success
vite-plugin-vue	✅ success
vitepress	✅ success
vue-i18n	✅ success
vue-macros	❌ failure
vuetify	✅ success
vueuse	✅ success
vue-simple-compiler	✅ success

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	85.9 kB (+4 B)	32.6 kB (+1 B)	29.5 kB (-3 B)
vue.global.prod.js	132 kB (+4 B)	49.3 kB	44.3 kB (-16 B)

Usages

Name	Size	Gzip	Brotli
createApp	47.9 kB (+4 B)	18.8 kB (+1 B)	17.2 kB (+55 B)
createSSRApp	50.6 kB (+4 B)	20 kB (+2 B)	18.2 kB (+3 B)
defineCustomElement	50.3 kB (+4 B)	19.6 kB (+2 B)	17.9 kB (+1 B)
overall	61.2 kB (+4 B)	23.7 kB (+1 B)	21.5 kB (-40 B)

[sxzz]

CI failed. Could you please format your files? pnpm run format

[KaelWD]

That adds a bunch of other changes in unrelated files.