### 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)

}
	updating_scroll = false;
	return total_height;
}
bool RichTextLabel::_validate_line_caches() {
	if (updating.load()) {
		return false;
	}
	validating.store(true);
	if (main->first_invalid_line.load() == (int)main->lines.size()) {
		MutexLock data_lock(data_mutex);
		Rect2 text_rect = _get_text_rect();
		float ctrl_height = get_size().height;
		
		float old_scroll = vscroll->get_value();
		if (main->first_invalid_font_line.load() != (int)main->lines.size()) {
			for (int i = main->first_invalid_font_line.load(); i < (int)main->lines.size(); i++) {
				_update_line_font(main, i, theme_cache.normal_font, theme_cache.normal_font_size);
			}
			main->first_resized_line.store(main->first_invalid_font_line.load());
			main->first_invalid_font_line.store(main->lines.size());
		}
		if (main->first_resized_line.load() == (int)main->lines.size()) {
			vscroll->set_value(old_scroll);
			validating.store(false);
			return true;
		}
		
		int fi = main->first_resized_line.load();
		float total_height = (fi == 0) ? 0 : _calculate_line_vertical_offset(main->lines[fi - 1]);