### 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)

const_cast<RichTextLabel *>(this)->_validate_line_caches();
	int tg = 0;
	Item *it = main;
	while (it) {
		if (it->type == ITEM_FRAME) {
			ItemFrame *f = static_cast<ItemFrame *>(it);
			for (int i = 0; i < (int)f->lines.size(); i++) {
				MutexLock lock(f->lines[i].text_buf->get_mutex());
				tg += TS->shaped_text_get_glyph_count(f->lines[i].text_buf->get_rid());
			}
		}
		it = _get_next_item(it, true);
	}
	return tg;
}
Size2 RichTextLabel::get_minimum_size() const {
	Size2 sb_min_size = theme_cache.normal_style->get_minimum_size();
	Size2 min_size;
	if (fit_content) {
		min_size.x = get_content_width();
		min_size.y = get_content_height();
	}
	return sb_min_size +
			((autowrap_mode != TextServer::AUTOWRAP_OFF) ? Size2(1, min_size.height) : min_size);
}
void RichTextLabel::_generate_context_menu() {