PlainEditor: cursor disappears when all text is deleted

[mwcampbell]

I can reproduce this in both vello_editor and in a Xilem single-line textbox after xilem#616. Based on the fact that in the latter case, the textbox shrinks, it appears that the layout also doesn't have a single line or run when there's no text. This also breaks accessibility. When there's no text, there should always be a single line with a single run, so there's somewhere for the cursor to be located.

[dfrg]

I believe #170 fixes this.

[mwcampbell]

It doesn't. In vello_editor running on the latest commit in #170, if I press Ctrl+A followed by Delete, the cursor still disappears.

[dfrg]

Oops, this was working but I pushed a change that broke it:

parley/parley/src/shape.rs

Line 46 in 7cae451

if text.is_empty() && !inline_boxes.is_empty() {

The condition for inline boxes is inverted.