Rule Proposal: `vue/require-render-return`

[armano2]

Please describe what the rule should do:

Enforces render function to always return value (require-render-return)

What category of rule is this? (place an "X" next to just one item)

[ ] Enforces code style
[x] Warns about a potential error
[ ] Suggests an alternate way of doing something
[ ] Other (please specify:)

Provide 2-3 code examples that this rule will warn about:

export default {
  render () {
  }
}

export default {
  render (h) {
    if (foo) {
      return
    }
  }
}