### Godot version

v4.0.2.stable.official [7a0977ce2]

### System information

Windows 10, not platform specific

### Issue description

Methods `get_character_line` and `get_character_paragraph` return wrong indices for characters that are first in the respective line/paragraph. See the attached screenshot.

![image](https://user-images.githubusercontent.com/892180/236546781-692603be-eb09-419e-822e-d4152a66d9b6.png)


### Steps to reproduce

Run attached project.

The script goes through all characters in `RichTextLabel` and prints the respective character, line index, and paragraph index. None of them should be -1 since we are iterating through valid indices.

### Minimal reproduction project

[RichTextLabel.zip](https://github.com/godotengine/godot/files/11409399/RichTextLabel.zip)

if (!is_visible()) {
		return 0;
	}
	const_cast<RichTextLabel *>(this)->_validate_line_caches();
	return visible_line_count;
}
void RichTextLabel::set_selection_enabled(bool p_enabled) {
	if (selection.enabled == p_enabled) {
		return;
	}
	selection.enabled = p_enabled;
	if (!p_enabled) {
		if (selection.active) {
			deselect();
		}
		set_focus_mode(FOCUS_NONE);
	} else {
		set_focus_mode(FOCUS_ALL);
	}
}
void RichTextLabel::set_deselect_on_focus_loss_enabled(const bool p_enabled) {
	if (deselect_on_focus_loss_enabled == p_enabled) {
		return;
	}
	deselect_on_focus_loss_enabled = p_enabled;
	if (p_enabled && selection.active && !has_focus()) {
		deselect();
	}
}
Variant RichTextLabel::get_drag_data(const Point2 &p_point) {