### 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)

Item *item = p_item;
	Item *prev_item = p_item;
	int level = 0;
	while (item) {
		if (item->type == ITEM_LIST) {
			ItemList *list = static_cast<ItemList *>(item);
			ItemFrame *frame = nullptr;
			int line = -1;
			_find_frame(list, &frame, &line);
			int index = 1;
			if (frame != nullptr) {
				for (int i = list->line + 1; i <= prev_item->line && i < (int)frame->lines.size(); i++) {
					if (_find_list_item(frame->lines[i].from) == list) {
						index++;
					}
				}
			}
			r_index.push_back(index);
			r_list.push_back(list);
			prev_item = item;
		}
		level++;
		item = item->parent;
	}
	return level;
}
int RichTextLabel::_find_margin(Item *p_item, const Ref<Font> &p_base_font, int p_base_font_size) {