### 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 st_args;
}
Control::TextDirection RichTextLabel::get_text_direction() const {
	return text_direction;
}
void RichTextLabel::set_language(const String &p_language) {
	if (language != p_language) {
		_stop_thread();
		language = p_language;
		main->first_invalid_line.store(0); 
		_validate_line_caches();
		queue_redraw();
	}
}
String RichTextLabel::get_language() const {
	return language;
}
void RichTextLabel::set_autowrap_mode(TextServer::AutowrapMode p_mode) {
	if (autowrap_mode != p_mode) {
		_stop_thread();
		autowrap_mode = p_mode;
		main->first_invalid_line = 0; 
		_validate_line_caches();
		queue_redraw();
	}
}
TextServer::AutowrapMode RichTextLabel::get_autowrap_mode() const {