### 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)

ERR_FAIL_COND((int)p_text_direction < -1 || (int)p_text_direction > 3);
	_stop_thread();
	if (text_direction != p_text_direction) {
		text_direction = p_text_direction;
		main->first_invalid_line.store(0); 
		_validate_line_caches();
		queue_redraw();
	}
}
void RichTextLabel::set_structured_text_bidi_override(TextServer::StructuredTextParser p_parser) {