Do text nodes with an empty string value generate a text run or not?

The difference will affect run-in layout:

If a run-in sequence is immediately followed by a block box [...]

And it's also observable in flow layout. According to CSS 2.2,

Line boxes that contain no text [...] must be treated as not existing [...]

I guess "text" refers to text runs. See https://jsfiddle.net/rs94bpay/

<div><div>Foo</div></div>

div {
  white-space: pre;
  border: 1px solid blue;
}

document.querySelector('div').prepend('');

Firefox does not discard the line because of the text node, Chrome and Edge do.