### 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)

return autowrap_mode;
}
void RichTextLabel::set_visible_ratio(float p_ratio) {
	if (visible_ratio != p_ratio) {
		_stop_thread();
		if (p_ratio >= 1.0) {
			visible_characters = -1;
			visible_ratio = 1.0;
		} else if (p_ratio < 0.0) {
			visible_characters = 0;
			visible_ratio = 0.0;
		} else {
			visible_characters = get_total_character_count() * p_ratio;
			visible_ratio = p_ratio;
		}
		if (visible_chars_behavior == TextServer::VC_CHARS_BEFORE_SHAPING) {
			main->first_invalid_line.store(0); 
			_validate_line_caches();
		}
		queue_redraw();
	}
}
float RichTextLabel::get_visible_ratio() const {