### 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 (tab_size == p_spaces) {
		return;
	}
	_stop_thread();
	tab_size = p_spaces;
	main->first_resized_line.store(0);
	queue_redraw();
}
int RichTextLabel::get_tab_size() const {
	return tab_size;
}
void RichTextLabel::set_fit_content(bool p_enabled) {
	if (p_enabled == fit_content) {
		return;
	}
	fit_content = p_enabled;
	update_minimum_size();
}
bool RichTextLabel::is_fit_content_enabled() const {
	return fit_content;
}
void RichTextLabel::set_meta_underline(bool p_underline) {