Position of cursor in empty lines doesn't respect the alignment

[ogoffart]

The position of the blining cursor on empty line doesn't respect the alignment.
To reproduce the bug, i added this line:

editor.set_alignment(parley::Alignment::Center);

in the Editor::new function at the beginning of examples/vello_editor/src/text.rs

As you can see, the cursor when editing a new line is all the way to the right instead of being on the center.
Starting to edit makes the cursor at the right position.

(Reported to Slint in slint-ui/slint#3580)

[nicoburns]

Definitely agree that this is a bug. I think it's just assuming that the caret position is the start position if there is no text to position the caret relative to. PRs welcome!