fix(slots): compiled slot fallback should be functions (#1030)

This avoids it being collected as dynamic children when it's not used.

fix #1021

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

@@ -15,8 +15,8 @@ export function renderSlot(
   name: string,
   props: Data = {},
   // this is not a user-facing function, so the fallback is always generated by
-  // the compiler and guaranteed to be an array
-  fallback?: VNodeArrayChildren
+  // the compiler and guaranteed to be a function returning an array
+  fallback?: () => VNodeArrayChildren
 ): VNode {
   let slot = slots[name]
 
@@ -34,7 +34,7 @@ export function renderSlot(
     createBlock(
       Fragment,
       { key: props.key },
-      slot ? slot(props) : fallback || [],
+      slot ? slot(props) : fallback ? fallback() : [],
       slots._ ? PatchFlags.STABLE_FRAGMENT : PatchFlags.BAIL
     )
   )