TreeBuilder-based layouts have invisible cursors when text is empty

Nobody has claimed this yet.

Even when the text is empty, a layout needs to have some proper line metrics, so that for example `Cursor::geometry(&layout)` can give a cursor of the correct size. `RangedBuilder` does this correctly, but `TreeBuilder` just fills the whole `LineMetrics` with zeroes. This results in an invisible cursor for any `TreeBuilder`-based text edit box when the text is empty.

This is a minimal example:

````rust
use parley::*;

fn main() {
let mut font_cx = FontContext::new();
let mut layout_cx = LayoutContext::new();
let style: TextStyle<[u8; 4]> = TextStyle::default();

println!("RangedBuilder with empty text ");
test_cursor_geometry_ranged(&mut layout_cx, &mut font_cx, &style, "");

println!();

println!("TreeBuilder with empty text ");
test_cursor_geometry(&mut layout_cx, &mut font_cx, &style, "");
}

fn test_cursor_geometry(layout_cx: &mut LayoutContext, font_cx: &mut FontContext, style: &TextStyle<[u8; 4]>, text: &str) {
let mut builder = layout_cx.tree_builder(font_cx, 1.0, true, style);

builder.push_text(text);
let (mut layout, _) = builder.build();
layout.break_all_lines(Some(400.0));

for (i, line) in layout.lines().enumerate() {
println!("Line {}: metrics = {:?}", i, line.metrics());
}

let cursor = Cursor::from_byte_index(&layout, 0, Affinity::Downstream);
let geometry = cursor.geometry(&layout, 1.0);
println!("Cursor geometry: {:?}", geometry);
}

fn test_cursor_geometry_ranged(layout_cx: &mut LayoutContext, font_cx: &mut FontContext, style: &TextStyle<[u8; 4]>, text: &str) {
let mut builder = layout_cx.ranged_builder(font_cx, text, 1.0, true);

builder.push_default(StyleProperty::FontStack(FontStack::from(&[FontFamily::Named("system-ui".into())] as &[FontFamily])));
builder.push_default(StyleProperty::FontSize(style.font_size));

let mut layout = builder.build(text);

layout.break_all_lines(Some(400.0));

for (i, line) in layout.lines().enumerate() {
println!("Line {}: metrics = {:?}", i, line.metrics());
}

let cursor = Cursor::from_byte_index(&layout, 0, Affinity::Downstream);
let geometry = cursor.geometry(&layout, 1.0);
println!("Cursor geometry: {:?}", geometry);
}
````

Output:

````
RangedBuilder with empty text
Line 0: metrics = LineMetrics { ascent: 17.104, descent: 4.688, leading: 0.0, line_height: 21.792, baseline: 16.0, offset: 0.0, advance: 4.1600003, trailing_whitespace: 4.1600003, min_coord: -1.0, max_coord: 22.0 }
Cursor geometry: Rect { x0: 0.0, y0: -1.0, x1: 1.0, y1: 22.0 }

TreeBuilder with empty text
Line 0: metrics = LineMetrics { ascent: 0.0, descent: 0.0, leading: 0.0, line_height: 0.0, baseline: 0.0, offset: 0.0, advance: 0.0, trailing_whitespace: 0.0, min_coord: 0.0, max_coord: 0.0 }
Cursor geometry: Rect { x0: 0.0, y0: 0.0, x1: 1.0, y1: 0.0 }

````

I blindly stumbled into a "fix" that looks like this: in `TreeStyleBuilder::finish()`, right before returning, do

```
if styles.is_empty() && self.text.is_empty() {
styles.push(RangedStyle {
style: self.tree[0].style.clone(),
range: 0..0,
});
}
```
This makes the style available even when the text is empty. I don't know if this can be a proper solution.

Start with TreeStyleBuilder::finish() and compare how TreeBuilder and RangedBuilder produce styles and line metrics for empty text. Reproduce the minimal example and inspect Cursor::geometry(&layout). Done means an empty TreeBuilder layout has nonzero line metrics and a visible cursor, with coverage for the empty-text case.

Written by the indexing model from the issue text.