fix(warn): avoid warning on empty children with Suspense

[posva]

The actual use case comes from Vue Router when doing:

<router-view v-slot="{ Component }">
    <transition mode="out-in">
      <keep-alive>
        <suspense>
          <component :is="Component"></component>
          <template #fallback>
            <div>
              Loading...
            </div>
          </template>
        </suspense>
      </keep-alive>
    </transition>
</router-view>

Component will be null initially (initial navigation) or when nothing is matched. The warning could be avoided by using <component v-if="Component" :is="Component" /> but I think having the warn could be confusing for users as they would be looking for a multi-node root they don't have

I also added missing tests for the existing warning

[posva]

I did this to make it Similar to Text, Static, and other existing symbols in vnode.ts. It could also be moved with the other existing vnodes

[github-actions]

Size report

Path	Size
vue.global.prod.js	42.56 KB (+0.01% 🔺)
runtime-dom.global.prod.js	28.16 KB (+0.04% 🔺)
size-check.global.prod.js	16.91 KB (+0.04% 🔺)

[posva]

This turns out to be possible so I added it

[jods4]

Would be nice to merge this PR if it's ready, or maybe keep the related issue #4016 open.
It was closed more than one month ago but it's not fixed in every release that happened since then.

[MisterDr]

Could you bump this one? So far this warning generated probably a few terabytes of logs.

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	86.5 kB	32.9 kB (+6 B)	29.6 kB (-67 B)
vue.global.prod.js	132 kB	49.6 kB (+1 B)	44.5 kB (+10 B)

Usages

Name	Size	Gzip	Brotli
createApp	48 kB (+6 B)	18.9 kB (-2 B)	17.2 kB
createSSRApp	51.2 kB (+6 B)	20.2 kB (-3 B)	18.4 kB (-2 B)
defineCustomElement	50.3 kB (+6 B)	19.7 kB (-6 B)	17.9 kB (+8 B)
overall	61.3 kB	23.7 kB (+3 B)	21.6 kB (+2 B)