Missing named slots throw error

[Jexordexan]

Version

3.0.0-alpha.6

Reproduction link

https://codesandbox.io/s/vue-3-named-slot-error-wovpr

Steps to reproduce

Open sandbox, check console.

What is expected?

Named slots skip render if content is missing.

What is actually happening?

Component doesn't render at all, and throws error.

Offending code:
https://github.com/vuejs/vue-next/blob/19a799c28b149b14e85d9e2081fa65ed58d108ba/packages/runtime-core/src/helpers/renderSlot.ts#L23

---

Stacktrace:

TypeError: Cannot read property 'length' of undefined
    at renderSlot (vue.global.js:9011)
    at Proxy.render (eval at compileToFunction (vue.global.js:10150), <anonymous>:11:7)
    at renderComponentRoot (vue.global.js:5027)
    at componentEffect (vue.global.js:6869)
    at run (vue.global.js:4031)
    at reactiveEffect (vue.global.js:4012)
    at effect (vue.global.js:3997)
    at setupRenderEffect (vue.global.js:6867)
    at mountComponent (vue.global.js:6860)
    at processComponent (vue.global.js:6786)

[underfin]

3375687

[yyx990803]

Oh I linked the wrong issue in that commit (it was meant to fix this one)