### 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 (text == p_bbcode) {
		return;
	}
	text = p_bbcode;
	_apply_translation();
}
void RichTextLabel::_apply_translation() {
	String xl_text = atr(text);
	if (use_bbcode) {
		parse_bbcode(xl_text);
	} else { 
		clear();
		add_text(xl_text);
	}
}
String RichTextLabel::get_text() const {
	return text;
}
void RichTextLabel::set_use_bbcode(bool p_enable) {
	if (use_bbcode == p_enable) {
		return;
	}
	use_bbcode = p_enable;
	notify_property_list_changed();
	_apply_translation();
}
bool RichTextLabel::is_using_bbcode() const {