feat(compiler): render <slot/> as block fragments

packages/runtime-core/src/helpers/renderSlot.ts

@@ -1,12 +1,25 @@
 import { Slot } from '../componentSlots'
-import { VNodeChildren } from '../vnode'
+import {
+  VNodeChildren,
+  openBlock,
+  createBlock,
+  Fragment,
+  VNode
+} from '../vnode'
 
 export function renderSlot(
   slot: Slot | undefined,
   props: any = {},
   // this is not a user-facing function, so the fallback is always generated by
-  // the compiler.
-  fallback?: string | VNodeChildren
-): string | VNodeChildren | null {
-  return slot ? slot() : fallback || null
+  // the compiler and gurunteed to be an array
+  fallback?: VNodeChildren
+): VNode {
+  return (
+    openBlock(),
+    createBlock(
+      Fragment,
+      { key: props.key },
+      slot ? slot(props) : fallback || []
+    )
+  )
 }