Suggestion: relax `invalid-template-root` for `v-if`

[HerringtonDarkholme]

Tell us about your environment

• ESLint Version: 4.0.0
• eslint-plugin-vue Version: 3.1.2
• Node Version: 8.0.0

Conditionally empty template root can be acceptable. For example, in a list item component, if an item is empty/undefined, empty template root represents skipping render.

The reason behind it is that we can encapsulate null check in the component, so we can pass empty value to the component.

vue-hackernews also employs such strategy.
https://github.com/vuejs/vue-hackernews-2.0/blob/master/src/components/Comment.vue#L2

<template>
  <li v-if="comment" class="comment">
  </li>
</template>

[mysticatea]

Thank you for the report.

I have not known that <template>s allow empty.
In that case, it's false positive.