fix(jsx-no-danger-with-children): account for ignored JSXText nodes

[marvinhagemeister]

Technically, JSX children aren't empty in cases like this:

<div>
</div>

Under the hood there is still a single child that contains only whitespace and is typically dropped during rendering.

JSXText {
  text: "\n   "
}

I forgot about that when writing this rule.

Fixes #1398