fix(runtime-vapor): properly handle dynamic slot work with v-if

[edison1105]

• template code

  <Comp>
    <template v-slot:[slotName] v-if="count <10">
      <h1>{{ count * 2 }}</h1>
    </template>
  </Comp>

• compiled code

  const n3 = _createComponent(_ctx.Comp, null, {
    $: [
      () => (_ctx.count < 10
        ? {
          name: _ctx.slotName, 
          fn: () => {
            const n1 = t1()
            _renderEffect(() => _setText(n1, _ctx.count * 2))
            return n1
          }
        }
        : void 0)
    ]
  })

the dynamic slot source result will be undefined if _ctx.count > 10, so we should ensure it is valid, otherwise slot.name will throw an error.