`prefer-stateless-function` should detect `null` in if shorthand return

[inyono]

With enabled rule prefer-stateless-function, the following does throw a warning although null may be returned:

class If extends Component {
  render() {
    const { condition, ...props } = this.props

    return condition ? <this.props.component {...props} /> : null
  }
}

If we write the result to a variable, though, there is no warning:

class If extends Component {
  render() {
    const { condition, ...props } = this.props

    const component = condition ? <this.props.component {...props} /> : null

    return component
  }
}