### 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 (visible_chars_behavior != p_behavior) {
		_stop_thread();
		visible_chars_behavior = p_behavior;
		main->first_invalid_line.store(0); 
		_validate_line_caches();
		queue_redraw();
	}
}
void RichTextLabel::set_visible_characters(int p_visible) {
	if (visible_characters != p_visible) {
		_stop_thread();
		visible_characters = p_visible;
		if (p_visible == -1) {
			visible_ratio = 1;
		} else {
			int total_char_count = get_total_character_count();
			if (total_char_count > 0) {
				visible_ratio = (float)p_visible / (float)total_char_count;
			}
		}
		if (visible_chars_behavior == TextServer::VC_CHARS_BEFORE_SHAPING) {
			main->first_invalid_line.store(0); 
			_validate_line_caches();
		}
		queue_redraw();
	}
}
int RichTextLabel::get_visible_characters() const {